well for one thing it can reduce dimming headlights, dimming headunits, power shortages, etc. its basically a back up power supply, it boosts your amp when the power isnt provided by the battery and alt. also it makes your amps performance better as in it can put out its full power. i would recomend getting one because for me, it made system i bit more powerful.
Caps dont really help out. The alternator is what basicly powers a car when the engine is running. Caps just store power but if your not getting enough power to begin with your lights are still going to dim. The only solution is to get a high output alternator. Myself I use caps just for spikes in my audio system. If for some instance there were a huge amount of current coming to your amp, the cap would absorb then release it to the amp.
what is the alternator in the car whr is it located i dont no much about cars or car audio sorry
The alternator charges your battery and provides power for your car, its like a generator. its going to be connected to your pulleys by the engine. you will see a coil of wires in . it depends on your car where exactly its located whethere its the top or bottom of the engine. Since your new to all of this you might want to go to a local audio shop for some help.
A capacitor stores a certain amount of power, depending on the capacitor. so when the amp pulls power, instead of pulling it all from your car, it pulls roughly 70% from the car, and 30% from the capacitor, give or take a few. which in turn makes sure your amp is pushing the proper amount of power each time. most cars can't provide all the power needed. so it will eliminate dimming headlights ect...but it will let your amp function to it's full potential. i would say it's worth getting, i havn't met anyone who has thought otherwise. hope this helps =)
for best use of a capacitor, it must be within as close to the amp as possible, run the thickest power wire too, i need to move to .0 gauge wire, but the guy who owns soundworld was tellin me that he did a paper on capacitors and for the best usage it should be right next to the amp, to know that its actually doing its job, the bass will hit and the voltage will drop on the cap's display