Supplimental PCI-E 6 pin power for gfx cards. Page 13 in your manual. Probably for use in quad SLI configurations.
I have 2 1080 in sli, i dont need do i? its not a problem i will just have to get an extra cable mod cable.
More than likely not. It's something I would ignore if it were mine and didn't have enough PCIe connectors. However, pg. 17 of the manual does indeed recommend using it for supplementing the +12V supply from the 24-pin cable when using SLI, CFX, and dual processor video cards.
I suspect though, that is required when populating all four PCIe x16 slots.