Your electrical devices not only draw power from the mains, they also have a nasty habit of depositing electrical noise back into it.
Why is this a problem? Well, your hi-fi and home cinema equipment is sensitive to this kind of noise, and electrical interference can upset all the work you've put into assembling your kit.
So a mains product that eliminates electrical noise is your new best friend. It's a good thing these blocks won't let you down.