What is the purpose of the rotating watch bezel?

The rotating watch bezel is a reference feature found in watches that is used to indicate elapsed time. It is a standard feature in watches worn by divers.

By rotating the bezel and indicating either the starting point of the minute hand, or the point where the minute hand will eventually move towards, a watch can accurately calculate the time elapsed during a dive. The modern rotating bezel is unidirectional; it can only rotate counter-clockwise and this acts as a safety feature in case the bezel is rotated accidentally. In such an accidental rotation, the watch would indicate more elapsed time rather than less.