Edit: Ah yeah...eggzackery what GT said...but more windy 
When the wiper motor is turned off at the switch, the two brushes inside the motor are shorted out by the switch to provide what is called an "Armature Brake". This shorting arrangement turns the motor temporarily into a generator within itself and causes a large shorting current to be formed by the still turning rotor which in turns rapidly decelerates the motor through the magnetic fields created. I'm not sure how it is needed when the wiper mechanism provides so much resistance to movement ...but anyway...
When the switch shorts the brushes, the ground is disconnected from the negative brush by the switch. If it did not you would have a dead short between 12volts and earth.
Their is a very large short term current created in the wiper motor to provide the braking and the wiper motor case may develop a rising voltage potential with relation to the chassis given the dodgy grounds through the rubber grommets and wiper plinths. Also, while the negative is disconnected (nearly always), then there is no path for the 12 volts to go to ground should the 12 volt supply to the wipermotor devlop a fault and short out the motor case. This ground wire dissapates this voltage. This is why the ground wire has no apparent funtional use, and why the negative to the motor has to be provided through the switch.
