The primary purpose of an automotive alternator is to charge the battery after it has supplied the high current needed to start the engine. Although many things will reduce the amount of available power in a car's battery, starting the engine provides the biggest hit.
This is due to the battery supplying 100% of the needed power for starting. Once the engine is running the charging system is responsible for producing enough energy to meet the demands of all the loads in the electrical system plus a little extra to provide a continuous surplus. Accessories like headlights, turn signals and Interior blower motors get their power after start up and are therefore powered by the charging system.