The third prong goes to ground. It's usually there as a safety precaution so that if the appliance breaks and a short develops, current will go to ground (the earth) instead of through a person. In a car the "ground" is usually the metal chassis, so a person touching a broken appliance and the car body would form a circuit, which at 115V 150W, could easily be fatal. Only having two prongs is a safety precaution.