What a capacitor is "supposed to do" is keep your line voltage at a constant. It helps keep your voltage from dropping too low. The only thing is NOT too many capacitors actually accomplish this feat and therefore are of NO practical use other than spending money on them.
BTW.....I use (2) 1 farad capacitors in my vehicle. I have them in line with my sub amps. Before I put those in I had slight head light dimming on big bass hits. Now I no longer have any dimming of the head lights what so ever. So in that case it worked. Truth be known an alt upgrade may have solved that problem for me too but I chose a path that included caps.