I noticed something interesting this morning coming to work. I was stopped at a light and my dash coolant temp light went out, so I figured I would turn on the heat. As I did, I noticed my idle speed increase. I hit the off button, and it dropped back down. I repeated the process a few times and sure enough, if the fan was on, the idle speed increased. My guess is they do it for one of two reasons. #1 would be to increase the water pump rpm, and get more heat to the heater core. #2 would be to increase the rpm to the alternator because the heater fan is pulling more power.