Basically - the heat energy needs to go somewhere. Say for example, we have two hot plates, both running at 80c. One has a fan blowing across it, the other does not. Heat plate #1 does not have a fan blowing air across it / away from it, meaning that it will dissipate the heat slower, and as such the plate it self will be hotter. Plate #2, whilst generating the same amount of heat as #1, has a fan blowing air across it- meaning the heat from the plate is moved around the room, and the plate itself will run at a cooler temperature. Very primitive, but should be simple enough. Basically, aftermarket coolers create the same amount of heat, but the card itself is generally cooler as the heat is transferred away from the card itself faster. Only way to decrease heat on the same board is reduce load / underclock, as water cooling will still spread heat.