Amp is plugged to the wall, why use a conditioner?

I keep hearing how important it is to have a line conditioner if you have higher end equipment. I also hear that I should plug my amp into the wall for best performance. If I get a line conditioner for my CD, DVD, and Control Amp to purify the signal, will it be defeated by the messy signal coming from my amp being plugged into the wall? Here are a few other questions: If I do use a line conditioner, should I get a few to plug into different wall sockets? Which conditioners do not limit my amp? Gray 400? Shunyata 2? Monster 2000 or 7000? When I do not have any music on, I can hear a low hiss or humm from my speakers. Will the conditioner help clean this up?

Thank you very much,
Greg