From your manual.
PCIE_12V (Power Connector)
This power connector can supply extra power to the PCI Express x16 slots on the motherboard. Connect the power supply cable to this connector when using two graphics cards. Failure to do so may lead to an unstable system.
So running one card you dont have to, two cards yes. My old Asus rampage 3 extreme had 2 molex connectors on the motherboard, the only time i plugged them in was when i was doing heavy overclocks on the GPU's. Was able to take a pair of 580's from stock core up to 998MHz with the extra molex plugged in. when i changed them over to a new board without the extra molex they could not come close to that speed.