Rotating outer ring
It is primarily cosmetic - the bezel was originally designed for diving, military activities or similar where counting down time was critical.
Before digital stopwatches etc were built into watches as a feature, what you did as a diver was turn the bezel to the time (say 30 minutes) of air you had left, then the minute hand would travel around the face to the position 0 on the bezel, telling you 30 minutes had passed.
You often find compass directions, various job specific calculators etc on bezels also, but unless you need a stopwatch / time reminder and don't have that feature in the watch itself, then it is purely a tradition to include a rotating bezel.
The original bezels would have the time markings going the opposite way to the watch face usually, however on many modern watches, they often go the same way, allowing you to see how much time has passed, rather than how much you have left.
Hope this clears things up :)
on Mar 31, 2007