For diminutive gadgets whose use case seems ultra-specific, dive watches provide a tremendous amount of utility. You don't even necessarily have to know how to dive to take advantage of your "diver" — you really just have to understand how to use the bezel, that ring around the watch's dial that turns and is covered in numbers.
So why does the bezel on a dive watch rotate, while many watches have fixed bezels, or none at all? Commercial dive watches developed in the 1950s in tandem with the then-new sport of SCUBA diving. Divers need to keep track of bottom time, as they have tanks on their backs with a fixed amount of breathing gas. Before the era of dive computers — wrist-borne computers that keep track myriad dive-specific information — a wristwatch was necessary for this purpose.
A dive watch bezel is typically the "count-up" type, meaning there are marks at 5- or 10-minute intervals ascending clockwise, and a "pip" or marker to indicate zero, which is typically aligned with 12 o'clock on the watch when the bezel is not in use. If, say, you enter the water and then reach deepest part of your descent at 12:30, you would simply align the pip, or zero mark, at 12:30 on your watch. This way, as the bottom time ticks by, you have a running gauge of how much has elapsed. (At 12:40, for example, the time will be aligned with the "10" marker on your bezel, meaning you've been on the bottom for 10 minutes.) Easy enough.
Divers also have to make one or more stops on the way to the surface after dives of a certain length of time, in order to allow nitrogen to slowly escape the body. The length of these "decompression stops" are calculated according to a table available to the diver. As the lengths of given stops differ depending on several factors, the watch's bezel can also be used in the same manner described above to calculate the time of these stops. If you have to stop for three minutes at five meters, for example, you can simply reset your bezel, as you'll no longer need it for measuring bottom dive during this dive.
Early dive watch bezels were bi-directional, meaning they were simply loose and could be moved in either direction. This wasn't optimal, as you could easily accidentally knock one'd bezel underwater and suddenly think you'd only been underwater for, say, 10 minutes instead of 20. Later on, bezels were fitted with a spring-loaded system that only allowed them to turn in one direction — counter-clockwise — such that the wearer could only ever over-calculate bottom time, rather than under-calculate it. (It's much better to believe you've been on the bottom longer than you actuallyhave and begin your ascent early than to have the opposite occur, in which case you can run out of air.)
Of course, a count-up bezel can be used in the same manner to time just about anything. They're especially useful when cooking, or seeing how long you can avoid checking eBay in one afternoon (you could time this with your phone, but the temptation to 'Bay would be too great). You can use a dive bezel to time any old task, and when combined with a water-resistant, shock-resistance package that's meant to be knocked around, it makes the dive watch one of the most supremely useless pieces of EDC that's carried over from the pre-digital era.