it allows more then 14.4 volts to travel to your system electronics, generally you would not want to do this unless you could isolate the rest of the car from your stereo. More voltage = more power and greater stability as the amp will not have to work as hard, however, it will put more wear on your internal parts.
It all comes down to what you are looking to do. I think external voltage regulators are good, for the purpose of stability, but not for letting more then 14.4 through.