At a bike fit that I had a couple of weeks ago I heard this explanation (he had a pedal stroke analysis computer hooked up to my bike - how cool is that). Around 90-100 rpm is the point at which you start using your slow twitch muscles - these are the endurance muscles that don't tire as quickly. Under about 90 rpm you are using mainly fast twitch muscles, so keeping a high cadence will allow you to go farther without tiring as much.