that's to make sure the internal amp gets the voltage taht it needs...
if it doesn't get enough voltage (most stock wires can['t carry enough) then the amp will run hot, not produce the full amount of power and could potentially fry.
Alright... guess I'll have to run a wire all the way to the battery. Do I HAVE to connect another fuse close to the battery? Cuz there already is one on the wire but it's closer to the deck.