It is a useful, if overpriced, device to help protect your gear from spikes in power that could damage it.
Serious power related problems can include a ground loop hum, not solved by the above, or serious brownouts and spikes as mentioned above. As far as dirty power somehow effecting the analogue wave that gets fed to your speaker to make it produce sound...... Some will argue but I fall in the camp that if you really have power that bad, you need an electrician, not a Monster Power Center.
Edit:
I also wanted to add that even if I just wanted a good power bar, I wouldn't buy Monster, if only based on their extremely shady marketing practices on similarly overpriced speaker and AV cable.
An excellent investigative journalism piece on the HDMI scam.
CBC's Marketplace - HDMI cables 
