Battery= 12 volts .....alternator =13.8 volts. By default, anything electrical runs at the highest voltage...which is the 13.8 (or more) volts the alternator runs at. Now when you over draw the alternator (which pretty much happens every time you draw huge current from an amplifier) the current falls back on the battery, which has plenty of power to deliver without a doubt, but that power from the battery is 12 volts and not 13.8v from the alternator, so when you over draw the alternator, you get severe light dimming when the current is taken from your battery. A cap is easily charged to 13.8 volts all the way up to 18 volts easily (unlike a battery)...in the brief moment your system goes BOOOOOM the cap will quickly provide the current required for that energy spike, preventing the lights from dimming, Obviously if you have an adequate alternator the cap should be charged up almost instantaneously ready for the next hit, but if you over tax your charging system, its like a beaver dam in the nile river...it will eventually fail. Electrical systems on their own are somewhat 'sluggish', so the big BOOM will dim the lights before the current can be delivered. Think of when you turn your lights on, or the cooling fan kicks on...the current falls down but then the engine picks up to compensate for the lost electrical charge. With amplifiers, a cap should buffer the light flickering out...
I have 2 Image dynamics 12's with two Memphis monoblock amps, total fusing = 200 amps...I run a slightly done up alternator (100 amps @ idle, 140 peak) and a 2.0F cap....the cap defiantly helps the light dimming, period.
