The Garmin GSC-10 cadence sensor I usually have on my bike won't read over 180 rpm. The longest I've ever sustained 180 rpm with the Garmin sensor is about 8 seconds.

I do have a PowerTap, and it has a built-in cadence sensor, but it seems to be a lot more accurate with longer-period cadence averaging than it is for short term peaks. The highest the PT cadence sensor has ever recorded for 1 sec is 231 rpm, but I'm not sure I believe it. 200-210 I could believe, based on how 180 rpm feels to me. 231 rpm is 50 rpm higher than 180, and I can't imagine being able to do that based on how hard it is for me to hit 180.

Why pedal that fast? Simple physics - power is equal to force times velocity, or torque times angular velocity for things that spin. One way to generate higher power is to pedal faster using the same force/torque.