Overclocking is always a great way to gain some extra performance and when you have a card that is as powerful as the GTX 295, the increase in MHz can really give some excellent gains. If you find yourself with an i7 series CPU sitting at around the 3.4GHz mark, you're going to have some pretty nice performance numbers at all resolutions.
The main thing we want to see when we benchmark is that we have a minimum FPS of 30; a few extra FPS is always appreciated, though. What you will find is that with no overclocking you might be able to play a game at high settings, while running at stock speeds and a resolution of 1920 x 1200. But the extra MHz on offer when you move to speeds in excess of 3GHz will allow you the ability to move those settings to very high.
Just because someone has the money to buy a GTX 295, it hardly means that they also have the funds needed to buy a CPU like the i7 965 which carries a stock speed of 3.2GHz and a massive price tag of 999.99 U.S. Dollars at Newegg, versus the i7 920 at 279.99 which is capable of offering performance greater than the i7 965's stock speed thanks to overclocking.
While you don't need to overclock your CPU to get good use out of the GTX 295, if you do you're able to get the most use out of it. For some people good is simply good enough, but for others they will want the most and that's where overclocking comes in handy for a card like this.