The gears you use also depend on the distance you are covering. I maintain over 90 rpm cadence most of the time. On my short, 3.5 mile commute, I sprint the whole way on the big chainring and small cogs, faster than hell. If I'm going on a 3 hour ride, I spin more on the bigger cogs, and maybe the medium ring.

In other words, you use gears to pace youself, along with the other uses mentioned already. To go far, use easier gears. To go fast on short rides, use hard gears.