The rubber band reduces noise and smooths out abrupt changes in speed when the motor starts or stops. Also, you can temporarily detach the motor by taking off the rubber band, which allows you to test if your gears can move freely.
Your new improved Sciborg should have two light sensors, as well as front and back bump sensors. Try to mount the light sensors such that their height can be adjusted easily. Extra credit will be awarded for the best solution to this mechanical challenge!