Exactly the same reason I am sitting with a couple of monster machines, most notably a top end AMD machine and a Top end Sandy bridge E machine. Just to run on the best, as you know cpu's don't affect gaming all that much, its only now that we're starting to see games making use of more than 2 cores, Games like BF3, Crysis 3, Metro Last light and so on.
Great example of this is Crysis 3. When the game launched the i5 3570K, AMD FX8350 and I7 3770K were neck and neck while running a GTX680. All sitting on the exact same fps. After update 1.3 the 3570K and FX8350 are still pretty much deadlocked but the i7 3770K gained a massive 15fps over those 2 cpu's.
Its pretty much just to run the best hardware out there and to get the best possible results.
There are people out there that believe a i3 3220 will actually bottleneck a GTX660TI or a 7950, that's why you also rarely see benchmark reviews with mid range cpu's or low range cpu's.
Edit: Maybe I am being a little mean towards Nvidia, I was actually looking for 15-20% increase from these cards but I should have known that a re-badge with new memory and slightly faster boost clocked wouldn't have brought huge performance increases.




