A watch is useful for two reasons: keep track of your pacing, and as a backup for the compass. (You can orient yourself with the hands of a watch.)

Altho lately I am using the GPS more to track my speed/packing since it provides three numbers - fastest speed, moving average, and overall average figuring in any breaks I take. (This is really easily skewed however - don't swing the GPS around on the lanyard unless you want to impress someone by showing them a top speed of 9 mph.)