The Bezel with first purpose holds the glass window and protects the watch from damage. But after that watch maker Integrate more useful function on it.
Watch bezel can be used for timing, monitor diving, speed, distance, determine heart rate and compass.
Normally you can use it in two ways
Measuring remaining time (count down)
Example, you go diving and your oxygen just enough for 25 minutes, you use rotating watch bezel to know when nearly end of 25 minutes to go above the water.
How to do it ? It is very easy. if the time is 11h20. Turn the arrow to 11h45. And when the minute hand go near the arrow you will know the time nearly end.
Measuring elapsed time ( count up)
Example : Before you start diving, turn the arrow on bezel to the position of minute hand. After that the position of the minute hand on the bezel will show the minute you diving. If the minute at position 25 on the bezel, it mean you have been diving for 25 minute.