More than likely that is not a power port but a diagnostic port used by Asus when they test RMA's. The reason Asus and other gpu companies add wires is to fit different psus. There are cheaper psus that have high wattage like 750w or 850w, but only have a 6pin and a 6+2 pin, so those included wires are meant to adapt existing wiring to fit the gpu. On no way, shape or form are they to be plugged directly into the psu. Each psu is wired different. It's the reason for specific pins, some are square, some 'D' shaped. On one psu, the 'D' could be a ground, and on another it could be a 12v.
On quality psu's, all of the power wires are the same, 18 Guage, its all that is needed. What you are thinking is quality wires from Asus, really isn't, its the same as the psu's, its just better looking to you.