This subject comes up from time to time, and we usually give the shortest answers possible. It is my intent here then, to give a little longer answer, in hope that some people will find closure to their questions, while others will just find it interesting. I am not an expert, nor am I a commercial diver. Several of my friends and recreational dive buddies are in commercial dive school and several already work in that capacity. The rest of this knowledge I dug up myself since I find the history and pursuit of underwater exploration fascinating. I am leaving any equations out of my ramblings below for the benefit of everyone.
The air we breathe is about 21% oxygen. At pressures higher than 1 ATM (air pressure at sea level) oxygen starts to become toxic. It affects the central nervous system and can cause convulsions and (usually) death. Humans have a physiological tolerance to this "oxygen toxicity". Some have a higher tolerance than others, but it is generally accepted that the safe maximum working depth when using air as a breathing gas is 200 feet. Below this depth, we would need a breathing gas that has less than 21% oxygen. Also, the remaining gas in the air we breath (approx. 79% nitrogen) has an interesting effect on us as well, narcosis. Basically, the deeper you go, the more drunk you will feel. This is debilitating, and underwater it can be just plain unsafe. Enter the need for a breathing gas that:
So, MOST people, if they are doing a scuba dive deeper than 200 feet will use Heliox as a breathing gas (actually they will use a gas more suited to recreational deep diving, Tri-mix, which is Helium, Oxygen, and Nitrogen - made by mixing helium and air). I say most people, because history shows us that there has been plenty of deep air diving beyond 200 feet, and it still goes on. Some people routinely dive beyond this depth on air, and even deeper than 300 or even 400 feet. There is no denying that these people have a high tolerance to oxygen toxicity or are nuts or both.
When diving with air as a breathing gas, nitrogen is absorbed (dissolved) into a diver's blood and body tissues. The longer you stay deep and breath the compressed air, the more gas your body is going to absorb. I'm sure you have all heard this before Surfacing must be done slowly to allow the gas to exit a diver's blood and tissues and be expelled back out through the lungs during normal breathing. With extended bottom times, this ascent must be so slow that decompression stops are taken at shallower and shallower depths along the way. In some cases, so much nitrogen has been absorbed by the diver's body that it can take hours of such decompression after a particularly long dive. If these deco stops are not taken, and there is a lot of nitrogen in the diver's system, then you can have a condition similar to opening a warm bottle of Coke. The gas comes rapidly out of solution, and foams up the soda (blood). This will kill you in most cases. When breathing Heliox, the diver's body still takes on the inert gas (gas loading) into his or her blood, but this time it's helium.
So far, I am still talking about Scuba dives, that is carrying breathing gas on your back in a bottle (tank) and blowing bubbles out into the water, never to see them again. The watch we are wearing is still only coming in contact with the water we are diving in, and nothing else. Any watch that has a 200 meter rating can withstand any scuba dive, and probably most with a 100 meter rating. There are actually standards out there for this, but I won't go through that here.
The advent of offshore oil drilling brought about saturation diving mostly. In the early 1960's, Jacques Cousteau was conducting a series of saturation exercises called Continental Shelf 1, 2, and 3 Conshelf for short. At the same time other organizations were doing something similar, including the U.S. Navy, but the Cousteau experiments were better documented for public consumption. Basically, they wanted to see if people could live for extended periods of time underwater, at great depths. Conshelf 3 was an underwater living chamber at a depth of 300 feet. The men inside lived there for 3 weeks, and conducted a variety of experiments. From a practical standpoint, they discovered that they could perform a variety of work on deep sea well-heads (again, oil drilling) and do it more quickly and easily than previous procedures allowed when working from the surface. They even went so far as to have dummy well-heads down on the sea floor so they could actually do the work that was theorized about. The diving was all done on excursions from the living chamber on supplied heliox from the surface. In the story about Conshelf 3 in National Geographic Magazine, it is mentioned that during decompression after 3 weeks at 300 feet a curious thing happened. A small "explosion" was heard coming from the sleeping chamber. It was discovered the crystal on Philippe Cousteau's brand-new dive watch had blown off at some pressure that was less than the 10 ATM that they had been living at for the past 3 weeks. Hmmmmm.
Physiologically, saturation diving works like this: Under pressure (at depth) the body takes on helium into the blood and tissues. Stay down long enough and the blood and tissues become "saturated" .that is, no more gas will dissolve into them. Say that at a given depth you will saturate yourself with helium in 24 hours. So after a 24 hour working dive, you have to decompress for say, 12 hours. Well, since you were saturated after 24 hours, if you stayed under pressure for a week, you would still only have to decompress for 12 hours, because the amount of gas you're trying to get rid of is the same.
So obviously, the economics of saturation diving come into play. It's cheaper to keep a diver under pressure for an extended period of time and have him work and live at that same pressure than it is to try and decompress him at the end of each work day. Sometimes, the bottom time for a day's work would require a decompression commitment that wouldn't get the diver back in the water the next day anyway!
A modern-day, very simplified version of a saturation dive MIGHT work like this: A diver goes into a living chamber that is onboard an oil rig. This chamber has his bed, his bathroom, etc. It is then pressurized to the depth at which the diver will be working, say 800 feet. At the beginning of each work day, the diver goes through a small door into a diving chamber or bell. He closes the door and the bell is detached from the living chamber by his surface tenders. The bell is lowered to 800 feet where the diver is to begin work. He is suited up, and is breathing surface supplied air. He opens the door in the bottom of his diving bell (remember the pressure inside the bell is now the same as the pressure outside) and exits into the water to begin work. At the end of the day, he returns to his diving bell, closes door to the water, and it is raised to the deck of the oil rig where it is attached once again to his living chamber. The diver opens the door between the two and cleans himself up. Since a deep saturation shift might last a month, the diver will be "stored" at this pressure until it's time to decompress him, each day returning to his living chamber at the surface. He will, however, always be under the same working pressure that he is at when in the water, and he will sound like Donald Duck whenever he talks. (Note that at 800 feet, the diver is probably breathing a gas that is about 5.5% oxygen, remainder helium.)
Finally Getting to Watches:
Whew. So you see the problem that the watch must deal with when it is with the diver in his living chamber and diving bell? Remember Philippe Cousteau's watch? If the diver has a watch with him in the chamber, it must have a valve through which helium can escape, or there is going to be a problem most likely an un-commanded crystal jettison. Rolex developed the first helium relief valve for a wristwatch, so we have them to thank for that. It was developed at a time when the wristwatch was the primary (or only) way a dive of any type was timed and we know that in general, watches are just plain useful. There is no telling what a commercial diver may have to time when doing deep work.
With regard to helium valves on luxury watches in this day and age, I can say this: Useless. I see the evolution, or how we got to where we are, of this watch feature like this: Rolex developed the feature, it had a specific purpose. Time went on and Rolex became the luxury item that we all know (I don't mean this as a bad thing). The helium release valve became a feature that people associated with high-end luxury sport watches (diver watches) and was something that further set the Rolex apart from its competition. More time went on and other high-end watch makers added similar features to a model or two of their own dive watches in order to play in the same field with the Rolex Sea-Dweller. A lot of speculation could take place here with regard to comparisons between Rolex and Omega and Breitling by the non-WIS public and choices they might make once presented with these different watch models by a salesman. It is not my intent to enter this debate here, only to say people definitely do make choices based on features, and what may be perceived as "better." The average Joe that has decided to buy himself a Submariner, goes into the shop and states such. He may even say, "I want the best dive watch there is, so I'm getting a Sub". The salesman's eyes may sparkle, and he says, "Well, actually, the Rolex Sea-Dweller is better. It can go deeper, and it has this fancy Gas-Release Valve for really deep diving." The average Joe may now be presented with a dilemma it may get worse if he decides to ask if any other watches besides Rolex have a similar valve for this type of diving. I could go on and on. The point is that it is now a feature that is used for marketing and sales, and falls into the same realm as a deeper depth rating. It really doesn't mean much. Never has the adage, "by what you like" been so true than with dive watches with He valves, because it really doesn't matter. It's all about style.
Personally, I think that a lot of people buy a Sea-Dweller because they like the look of the Submariner, but want something a little unique. If someone notices their watch and says, "Hey, nice Sub.", they can smile and say, "Actually, it's a Sea-Dweller." I think there is a lot of appeal in that. As far as the Omega Seamaster is concerned, the valve is a prominent feature, and is most likely meant to get people talking. If someone sees one of these watches on a friend's wrist and questions the extra crown, he might walk away thinking, "That's cool", and presto a new customer is born. I like the design of the Seamaster, and the crown at 10 doesn't bother me, even though the feature itself is useless.
So take that as you will, and remember that the only thing you should be concerned about when deciding on what luxury dive watch to buy, is how it looks.