Cliff Notes:
A cap absorbs electrical spikes by bulding stored energy during quiet periods, and dumping them during short sudden bass attacks, thus showing your electrical system a more consistant (but not lower) electrical draw.
The 'not lower' is the key. If your electrical system already can't keep up and your headlights are dimming, idle is stuttering, etc. etc. it wont help.
I personally use one, mainly because it came with my wiring kit, but my alternator can keep up with my system, but i'm definitely maxing it out //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif So if i can keep a little strain off my alty, it's worth every penny.
Tricky thing is, you wouldn't know if you needed a cap, because you never NEED one, it's more just to reduce sudden stress on your electrical system.