A capacitor only holds the charge for a split second, so when the bass hits it pulls its initial power from the capacitor, then the capacitor has to pull the rest from the battery anyway. So its pointless in most situations.
As Migz said, it will smooth the load on the battery and alternator by providing a small reservoir of current. It shouldn't really be needed as the battery is huge capacitor and should be able to supply more than enough current but thats not the way things work out in the real world :/ A lot of load changes (ie. flashing headlights) will wreck some alternators quickly so even if you don't need capacitors it would be fairly cheap insurance to fit them.
because theres a ton of inductance bewteen the alternator and battery in the front of the car and the amps in the rear and you need to offset that with some capacitance to be able to draw current spikes
Its not actually made for what people think its made for, its not supposed to help the drain on your system or any of that crap, its just what a lot of people use it for which is actually bad for your electrical supply.
They are made to clean up voltage spikes in your car. If you read the link i provided before it explains that.
DO NOT GET A CAPACITOR. It is not needed in this situation, it will do more harm then good.
Spent the last year and a bit trolling the Internet about car sound systems, and ive read about capacitors more times I care to remember. Not to mention I used to have one, and my voltage use to actually be worse in my car after I'd put it in. I'd get a slightly bigger voltage dro with it.