ahhhh power caps!!!
Basically Caps store charge, same as a battery. As a cap is much smaller than a battery it will discharge in milliseconds as opposed to a battery which takes longer. The theory that they will sell you is that as a cap discharges faster it will supply "much needed" power to the amps faster than the battery can which can "improve bass response". All nice in theory but doesn't mention that after the discharge the cap charges again from the power system so in effect (sort of) delays the power drain through the system as the amps draw from 2 supplies for a millisecond. If you have a decent battery and good cabling all round a cap will have a detrimental effect on your system as it draws power from it.
Another point is that all amps have suitible caps built in anyway so even less need for an external one.
they can also solve problem with lights diming, but the power cap should only be used as a last cure 4 the problem.
basicly people put them in to look gud. its more than likly that they didn't need them, did ya see n e of the sound off cars with them???