A cap regulates the fluctuation in voltage to a certain degree.
The theory is that it will hold your voltage up at 14.4 for a split second while the bass note first hits, and the amps empty the alternator.
The down side is that they will hold the voltage "down" at 12 for just as long or longer.

The headlights will still dim and brighten, but they won't "flicker" as quickly.

In some cases, with certain music, they can help a stuggling alternator maintain a bit more punch.
For long sustained loud notes, a cap will be a detriment....most SPLers find that they hurt their scores with the 3 second burps they need to make the meter register.