This is a high end PSU. The 1080 and the 6700K are not high end components. Many systems require larger PSUs for any number of reasons.
Running actual high end cards (Which float in the TDP range of 300 or more watts) a high end CPU (The Enthusiast lineup with higher TDPs) RAM, HDDs/SSDs, Fans, Lights, Water cooling pumps, drives, the list can go on, require large amounts of power. If you OC, which many users do these days, the power requirements can go up 50% or more.
You can very easily hit 1000-1100 watts on a true high end rig.
Also, as stated above, there is nothing wrong with having a higher end PSU that will use less than its maximum rated power output. All throwing a 1200W PSU into a rig that needs only 500-600 will do is make the PSU more efficient, and have a longer life. On top of giving you the overhead for plenty of growth and expansion.