If the battery hasn't quite gotten "up to snuff" -- or if it's in a "discharge state" (as it is when you first start your car) -- the alternator provides electricity, through the voltage regulator, to charge up the battery. This means that the hardworking alternator has to power your accessories and the distributor at the same time as it charges your battery. The voltage regulator prevents the alternator from charging the battery above its normal level; this protects the battery.