A turnable bezel.
Tutima Pilot's Chronograph 1941 - classical pilot's watch with turning bezel
With pilot's watches the bezel is turning in both directions and has a decisively visible marker.
The clockwise turning permits to measure or check set time intervals.
Diver's chronograph Sinn U1000 S with diving bezel
With dive watches the bezel is turning unidirectional (counterclockwise). Before the dive, the diver turns the zero mark of the turning bezel to the position of the minute hand. While looking at the watch he can then immediately recognize the diving time passed off. The reason for the counter-clockwise adjustment only lies in the security thought If the turning bezel is accidentally rotated during the dive, then the start of the dive seems to have been earlier, and so the diver is always on the safe side when taking the decompression into account.
A rarer form of turning bezel is the slide rule bezel, as it is typical for many models of Breitling, eg the known model Breitling Navitimer. Here, the bezel has a logarithmic scale. If the turning ring is displaced relative to the fixed part, arithmetic operations such as multiplication or division are possible (addition means multiplication, subtraction means division, see: explanation with examples).