Enermax Galaxy 850w cable quesiton

[∞Velocitymaster∞]

I have one strange 3 pin cable, looks like for fan. I looked at the manual and it says that its for fan RPM only. So which fan is going to control? The 2 PSU fans? I know this plugs on the 3 pin fan power on the motherboard.
This what's written (FAN RPM For 8cm fan RPM detection.) I don't get this can someone explain. Also do I even need to worry about, I haven't have this plug in yet.

 

[Rabid Badger]

It doesn't control anything, it just allows your motherboard to know the speed of the fan in the power supply.

As for which fan, it will most likely monitor the fan on the back of the PSU.

No, it's not essential.