If you actually understand what a capacitor is used for then you're 2 steps ahead of half of the people who have posted opinions on them.
A capacitor is used to smooth out power distribution, this is why they are used in all power supplies. An AC/DC power supply is a whole other animal, but the basic principles still apply. the rectifier (or battery and alternator in car) never sends a smooth 0 Hz signal. there are "bumps" in the signal, and the capacitor acts as a filter to remove those "bumps."
This is about the best way I know how to simplify what a Cap does. I have worked in car au