It's a question of where the work gets done. When you spin (high cadence) the heart and lungs do more of the work. When you mash (lower cadence) the legs do more.

In terms of a smooth mechanical motion (as mentioned by orbthorn), if you live somewhere with hills, you need a low(ish) gear to get up the hills, that means you've got to be able to spin smoothly at high cadence to get back down at speed and to move quickly on the flats.