Originally Posted by raymor

The same reason you need one in a gas car ir a bicycle - to keep the motor at an efficient (or powerful) RPM as the speed of the vehicle changes.

You have less need for very low gears because electric motors make max torque at 0 RPM. Also the gearing is a bit different because electric motors are most efficient at 50% RPM while gas is most efficient at 75%, but still you need gears to reach the correct RPM.