Q: different powered chargers

I notice that there are three different powered chargers that i can buy, 45W, 60W and 85W. Is it necessary to use a certain powered one depending on what mac Mac I am going to be powering? Or can I use an 85 watt one with any Mac? If so, why would they even sell the lower power one (I am assuming that and 85W charger would charge twice as fast as a 45W charger...)?