[Tommy]

The battery is more than for starting the car. Think of it as a large reservoir of electrons than can be drawn on any time the alternator is not producing enough to meet your car's needs. That can happen at low RPM when the alternator is producing little or no electrical output, and it can happen when very high loads (e.g., headlights, electric fuel pump, electric fan, windshield wipers, stereo, etc. all on at the same time) exceed the capability of the alternator.