I just put two more fans in my computer case last night, and I noticed that along with the power cable on the fan, there was also a small 3 pin cable that can plug into the motherboard. There are two places for extra fans to be plugged into my motherboard, and I only have two case fans, so I don't know whether I need to plug them in or not. Right now they're just plugged into the power supply and they're working fine, so what's the purpose of plugging them into the motherboard? Thanks.