depends on what you are talking about...if you are talking about the MagFloat, it is used to clean the inside of your aquarium (put the rough side inside the aquarium and the smooth side outside...it acts as an algae scrubber).
Some people use magnets to keep their powerheads in place...I haven't tried this yet, but I would have to think that it would work better than suction cups if you have strong enough magnets.