Scuba divers only need to know how many minutes they have used.
Set the bezel pointer to where the minute hand is, you'll know how much time you have spent underwater. Knowing how long your tanks can last you can decide when you need to surface. The uni-directional is a fail-safe device, if you accidentally move it, you can only move i counter-clockwise. This means it will show that you have spent "more" time under water. As a result, you would surface earlier if you are purely depending on the minute hand. If the bezel could move clockwise, you might end up longer underwater.
As a point of interest, there are also count-down or elapsed time bezel where the numbers 0-60 run counter clockwise. Here you decide how much time you want to spend under-water, eg 10 minutes, set the bezel at the 10 minute mark and the minute hand will head progressively down to zero. Search Google images for 'count-down bezel' or 'elapsed time bezel'.. unfortunately I don't have any of these but alsways thought that made more sense for a diver.
Anil