Because 7970 Ghz edition wasn't enough
Seriously though, need it for Oculus rift and got a great deal - 500 euro unopened.
It's really good too, faster than Titan X at half the price and overclocks pretty crazy as well.
Still not enough to max out GTA:V and Batman: Arkham Knight at stable 1080p 60fps, need SLI or next gen for that.