>>1586516It's for diving. The easiest way to explain it is imagine a simple kitchen timer, where you would twist it to your desired minute count where it'll start counting down.
On a watch with a rotating bezel, the "dial" is the minute hand (which you can't move, but moves on its own conveniently in one minute increments), and you rotate the outer case of it.
So no matter where the minute hand is, you rotate the bezel so your desired time limit (let's say 15 minutes) matches the minute hand. If your watch reads 12:10, you don't want to try and math that you'll die at 12:25. Instead, you rotate the bezel so the 15 is right above the minute hand at ten, so rather than mathing the difference between 12:10 and 12:15, you see that the little arrow is now pointing to ten minutes left, then five, etc.
a $10 casio renders all that shit obsolete though.
