It’s worth a try: Does anyone know the purpose of watches that have a ring around them that you can turn. Usually they have marks that line up with the numbers on the dial, and usually one mark is different from all the others. It usually will only turn in one direction
I would have though it might be for setting an alarm, like it was on one of my old watches, but that doesn’t seem to be the case here.
primitive stopwatch, basically.
I have an old dive watch that has a moving bezel to measure air reserves. It can also be used to measure speed, and is called a tachymeter.
It’s to show elapsed time. You set the mark where the minute hand is right now; when it reaches 10 your egg is done.
I usually wear a GMT Master II. (Only it looks like the one on the left instead of the one on the Wiki page.)
The GMT Master has a 24-hour hand on it. In the original watch, the bezel could be turned so that the 24-hour hand could indicate a second time zone. For example, it’s 0800 hours in San Diego and you need to know GMT for your flight plan. Simply rotate the bezel so the 24-hour hand points to 1600 hours. You fly to New York. To get GMT, just turn your bezel back three hours. The GMT Master II gained a quick-set hour hand, so the 24-hour hand was no longer slaved to the hour hand. The means that GMT can be set directly and there is no need for the turning bezel. (This is how mine is set.) However the bezel can still be used to indicate a third time zone.
The Rolex Submariner has a 60-minute bezel instead of the GMT Master’s 24-hour one. As yoyodyne says, this shows elapsed time in minutes. It is used as he describes.
My Seiko Sports 100 Chronograph has a tachymeter bezel, as illustrated in Shark Sandwich’s link. Very useful if you need it, but I’ve never needed it.
This Seiko Bell-Matic is not quite like mine, but it’s similar. Its bezel (if you will) is internal, and is set with a knob. Its primary function is to set the alarm time. It can also be used as a countdown gauge. Unlike the Submariner’s bezel, which counts elapsed time (it counts up), the Seiko’s scale counts time remaining (counts down).
A watch is a tool. Like any tool, it has special features suited to its use. The pilot wants GMT at a glance, so there is a 24-hour bezel (and hand). The diver wants elapsed time, since he knows how long he can spend on the bottom and can easily read the elapsed time on the 60-minute bezel. An engineer who didn’t happen to have his favourite tool can turn to the tachymeter on his watch to count cycles and such. (Ditto the track-and-field coach or race car driver.) So the ‘purpose’ of the bezel depends on what it was designed to do, and what the user wants to use it for.
.
Folks are mixing two kinds of bezel rings here.
The most common is the rotating ring that has minute markings. The Rolex Submariner is an excellent example of this.
This ring is for dive times, and it typically has a click ratchet that prevents rotating it in the dangerous direction (the direction that would make you think you had more air than you really had).
The other common type is a fixed ring that has a tachymeter scale. This kind is often used for counting units per hour, such as miles per hour.
Here’s an example of how to check your speedusing a tachymeter.
Besides these two already mentioned, there are also circular slide rules that let you do fancy calculations. Here is a Citizen watch with one of these. These are more trouble than they are worth IMHO.
ETA: Johnny got more and better links.
I have an E6B for that. 
The unidirectional bezel is designed with a specific purpose. It is intended to allow a diver to measure their bottom time, or air time. Once at depth you set the zero marker to align with the minute hand. Then the minute hand against the bezel reads off the time you have spent. The critical design component is that the bezel is unidirectional. The bezel has a spring detent that holds it in place, but a bump to the bezel can only move it one direction - the direction that will add time to the measured time. Thus the watch behaves safely. An accidental bump will result in you spending less time at depth, not more.
Interesting. As far as I know, my mom’s watch was not designed to be a diver’s watch. (Did I forget to mention this was my mom’s watch?). It just does not appear to be well crafted enough for any real depth of water.
Also, one of the movable rings on a watch I found had compass directions on it. I assumed it was for that old fashioned way of using a clock and the sun as a compass. What was weird is that that particular watch also had a liquid compass in the band.
Isn’t this what they’re using in movies when they say “synchronise watches” (without specifying an exact time to which they are now setting them)?
the bezel for elapsed time in minutes also was featured for parking meters and phone calls, this on watches that were waterproof to 10 feet.
No. Many watches, particularly chronographs, will reset the seconds hand to 12 when you use the sychronization feature (or just reset the watch).
As noted above, the single purpose of a bezel is to measure elapsed time, either as a countdown, to monitor time remaining (as in following a dive plan), or for comparison to a scale (tachymeter). However, on modern watches it is mostly decorative as most people don’t know how to operate the timing/tachymeter features, and on many cheaper watches it doesn’t even move.
As for “professional dive watches” I haven’t seen an actual professional diver use one in years. Nearly all divers use dive computers today, and even those prudent divers who wear a mechanical backup timer and depth gauge usually have a dedicated timer for that, as sticking your Rolex outside your wetsuit not only requires installing a band extender or different band but also exposes your multi-thousand dollar piece of jewelry to damage and loss. I question that pilots use the slide rule for calculations that can be easily done with a pocket calculator, and I know that professional drivers don’t sit and fiddle with their watch while streaking down the speedway. I have used a watch tachymeter to calculated speed/distance in yacht racing, but only because it is really boring being the bowman on the downwind leg once the spinnaker is set.
Stranger
Huh. That’s strange. I could have sworn that my Submariner had an expansion link that would allow it to be worn outside of a wetsuit when deployed. But it doesn’t. Maybe the expansion link was on the older one I used to have? In any case, I know there are bracelets that allow a diving watch to be worn on a bare wrist, and that have an extra 30mm or so link that folds out so they can be worn with a wetsuit. As for damage, I’ve seen some Subs that were beat to hell (including missing the bezel) by divers. These were watches that were made when they were just tools, instead of an accessory for your MBA and BMW.
I’ve been around pilots all my life, and those that have the slide rules on their watches seem to use them just to impress people. For actual calculations they use their E6B, or else they have an electronic E6B. Personally, I’m more comfortable with the traditional E6B, since my electronic one draws a lot of power and was usually dead when I wanted to use it. Or I’ll just use a pocket calculator. 
Aha! I found it. The Sub has a 10mm expansion link that I’d overlooked before. (It’s been a while since I’ve been diving, and it’s cold enough for a dry suit up here.) The longer link I ‘remembered’ must have been on a different watch. In any case, the Sub can be worn with a thinnish wetsuit without changing the bracelet.
Rotating bezels are applied to many types of watches these days to give them a “sporty” look. On some really cheap models the bezels don’t even rotate at all, they are fixed in place.
I have a Citizen watch fitted with a circular slide rule for calculating fuel use, unit conversions, distance-time-speed calculations and so on. In practice I never use it because the numbers are too small to focus on in a noisy vibrating cockpit environment, and because it’s operated by turning a fiddly knurled knob on the left side of the watch. So you can’t really use it while you are wearing it.
Having said that, I never use a pocket calculator for these calculations either. On the ground, a laptop. In the air, the (plastic, non-electronic) flight computer.
I see that you found it, but for the curious, here it is (top right).
They made the extension quite stealthy, to the point where you wouldn’t know your own watch had this feature without someone showing it to you.
To extend the bracelet, you press in on a very subtle circle stamped on one of the links under the clasp, seen in the second photo from the right on the top.
This watch, and others like it such as the Omega Seamaster Pro with bracelet extension, are serious about their usage by divers, even if most wearers never get in deeper water than a swimming pool.
It wouldn’t surprise me if the humble handheld calculator takes a back seat to paper forms and plastic circular slide rule gadgets. Not being a pilot, I imagine that most pilots would either use dedicated plastic or metal slide-rule devices such as Johnny mentioned, or some fancy-schmancy bit of flight technology that computes everything for them.
When I was trained in naval nuclear power as a specialist in water chemistry and radiation control, there were dozens of calculations that were needed on a regular basis in order to determine chemical additions needed to keep plant parameters in spec or to compute radiation exposures based on certain factors.
For many of these there were charts and nomographs. For more complicated activities, there were paper forms that were filled out by hand, with all of the calculations written on them.
Tools like charts and nomographs have some advantages over a calculator: they are analog and purpose-built, so most of the time if you make an error, your result is fairly close to the correct answer anyway. If you make a calculator error, it is quite easy to be off by several orders of magnitude.
In addition, a fancy chart can simplify a fairly complex calculation and is nearly foolproof and never breaks. Ours were printed on some kind of weird super fiberglass paper stuff that was indestructible.
In the case where calculator action was actually needed, the steps were lined up in pre-printed forms with the actual computation printed in baby steps between each blank. You would pencil in your measurements in the blanks and then carry out the calculations.
I had one like that too and agree that the slide rule was of no practical use. In the air I just use a calculator.
