While everything in the car is 12 volt, some electronics pull more amps than others, and you need to be able to supply the car with enough amps to power everything. This is the best analogy I can come up with. Think of it like a giant water wheel. The water wheel needs a certain amount of water pressure to make it move (voltage), but it also needs the reservoir of water to feed it (amperage). So your voltage is the amount of pressure the water is exerting on the wheel, and the amperage is the reservoir. If you add an aftermarket stereo, or anything else that uses more power, its like making the wheel wider, it still needs the same pressure to move, but that pressure needs to be spread over a larger area requiring more water. So, your using more water from that reservoir meaning it will drain faster, so you need to be able to fill it faster. Or in the case of your car, you need an alternator that provides more amps. Hopefully that makes sense.
COTY - 2012
Tom - 03' Bonneville SSEi', 00' Sierra SLE, 1980 Goldwing GL1100 Interstate