A cap is very good to have, especially if you're running a high powered system. A capacitor works by using it's stored energy to supply the amp. Say you have a bunch of really fast, heavy notes, it's gonna take a lot of energy to supply that amp. Instead of pulling it all form your car's battery, it takes it from the capacitor making it easier on the battery. Then all the battery has to do is slowly "refill" the capacitor.