The traditional bezel is like stop watch, that is, you can set the 0 where the minute hand is, and as the minute hand moves forward, it counts up and you can read how many minutes have passed since you set the bezel. A reversed bezel, on the other hand, counts down like a timer. If you want to set a timer to 20 minutes, you can set the 20 min mark to where the minute hand is, and it will count down and you can read how many minutes are left until the timer is up.
In my opinion, the timer is far more useful than the stop watch. I find that I want to count down far more often than I want to count up. For example, if I want to cook something in the oven for 20 minutes, I can do that with a reversed bezel. But certainly for diving, it would seem that the reversed bezel is exactly what you want. I've even heard that back in the day when they actually used dive watches for timing the amount of oxygen you have left, they actually did use reversed bezel. You can still of course, use the normal bezel for keeping track of time, just like you could use a stop watch to cook your food even though it isnt really meant for that like the timer is. But you have to remember how long you were supposed to stay underwater, where as with the reversed bezel, you can just set it and forget it, and you know that as soon as the minute hand reaches the pip you need to come up. And you can look at the bezel at any time to see how many minutes you have left, where as with the diving bezel you have to do some math in order to find out how much time you have left.
Now I understand that nobody actually uses them for that nowadays, they have special dive computers and the watches are more of a novelty than anything. But I still cant see many situations where I would rather have the normal bezel instead of the reversed. Certainly you would think that there would at least be some kind of a debate over personal preference yet you hardly ever see watches with reversed bezels. And when you do they are usually on flight watches for some reason. I feel like I am the only one that understands this, everyone else seems to look at a reversed bezel watch and think that it is completely useless.