For most the GTX 580 by itself is going to offer more power than they need. Combine two of them and you've just got an absolute mammoth amount of performance on offer due to the excellent scaling we're seeing with dual card technology these days.
Take those two cards, though, add some extra voltage to the cores and move the clocks of both the core and memory north, and you're just talking about a huge amount of performance that makes any game playable at any single screen resolution, and while a lot of people say that the benefits of AA aren't nearly as strong as we climb up to these massive resolutions, there's still a difference and what this kind of setup does it give you the ability to make sure you can crank up every aspect of detail.
At 1920 x 1200 with 4x AA we can't get a single card to offer us a 60 FPS average, which is exactly what most gamers are going to aim for. People who have opted for a 120Hz monitor instead of the standard 60Hz offering are going to want even more with FPS averages closer to that 120 FPS mark.
With the GTX 580 in SLI and overclocked, we're only three FPS away from an average of 100 FPS and while our 980X at 4.2GHz might be overkill for most people, much like this setup, if you're dropping more than $1,000 on video cards the chances are you're going to have a rig running similar clocks to us, if not higher.
If that wasn't enough for people to understand the reasoning behind why people buy setups like these, you can then of course bring multi monitor setups into the mix which opens a whole new need for power.
Sure, you've got some insane power on offer here, but the bottom line is it's power that people are able to make use of if they play the right games or run the right resolutions. If you're on a single 1680 x 1050 screen it's no doubt overkill, but if you demand FPS in excess of 100 for that 120Hz monitor you've got or want maximum detail at 2560 x 1600, this is going to be the kind of setup that grabs your attention and that's why you go this setup and then proceed to overclock it. Because as we all know, you can never have too much performance!