If the power center has battery backup as well then it will correct large sags in voltage as well as brown outs and short blackouts. Often times when the power comes back on after a brown/black out there is a huge spike that accompanies it. A voltage regulator will protect against that.

Another benefit is its surge suppression capabilities. The components inside the PowerCenter will react faster than the cheap power strips on the market that only have MOV (Metal Oxide Varistor) protection.

Most of all though it just looks impressive and provides a central place to plug everything into. I have a similar item from a different manufacturer (Belkin) and I like it. It may also provide a trigger out connection so that when you power off your receiver some of the outlets will power off. Very handy if you have amps or you want to leave your sub on instead of having in auto mode (with auto mode there is sometimes a momentary lag before it kicks in and it may shut off if no LFE are sent for a while during a movie).

They also sometimes provide a front outlet so if you bring down a game system or camcorder you can plug it into the front rather than pulling stuff out from the wall.

One thing to consider though is that retailers make a killing off Monster stuff. There is a lot of markup on it. You should shop around for better prices and different manufacturers. Some other manufacturers are Panamax, Furman, Belkin and APC.

EDIT: As per Big Q’s answer, only some (non battery ones) can increase the line voltage and only for very short periods of time (milliseconds), most do not.