Your car's electronics pull from the battery in your car. The alternator which runs directly off of the main serpentine belt, keeps your battery charged as you drive. In short, it supplies electricity to two main things that keep your vehicle operating. The computer and battery in your car.
The computer is the brain of your vehicle. The alternator is like the heart in your body which keeps a steady supply of power to it. As your battery drains while driving, then the alternator runs to keep the voltage regulated. It is important for the voltage to be maintained as many problems can occur if your alternator fails.
When you crank your car, the alternator starts and begins supplying energy to the aforementioned components. You can also think of the alternator as a separate small electrical engine being run by the turning of your main engine.