http://en.wikipedia.org/wiki/Activated_carbon
Read up. In our aquariums, it has it's uses. You don't have to have to running in your filter all the time, it's just a waste of money. It's nice for making water a little clearer (in most tanks this isn't needed), but in tanks with a lot of wood and plants, it helps to clear out tannins and stuff. It's also nice to get rid of medication the water if you are done treating a sick fish.
Save yourself some cash, don't do it.