The header will almost certainly be used by board partners for custom editions of the GTX 1080. Should AIB partners wish to build a GTX 1080 using the reference PCB, but with an additional power header (6- or 8-pin, from the looks of it), it'd seem that nVidia has already made that allowance on its reference board. This would be useful in overclocking scenarios where power becomes the limiter – something we suspect to be one of two OC throttles for the GTX 1080. The other suspicion is thermals – but we're testing that today.
Regardless, the header mounts at a 90-degree corner to the reference FE board. We've emailed a few AIBs to ask them if they intend on using this header anytime soon and will update if we hear back.
Update: Spoke to some manufacturers. Sounds like it is because the same board will be used for professional series cards (e.g. Quadro), and those cards will run beefier power configurations. AIB partners have told us that they're running their own board designs as the reference design may not survive an additional 8-pin header (could damage the VRM).
Editorial: Steve “Lelldorianx” Burke