I think the main purpose it serves for Nvidia is branding. This gets plastered all over the tech news and everyone knows Nvidia makes the fastest GPU. People then associate them with better technology (for better or worse).
Why someone would buy it? There are a lot of people who want the very best. It's the same reason people buy an Aston Martin over a Toyota.
From a more practical perspective though, there's also future proofing. This card will be able to deliver high frame rates for a few generations of games. People who don't want to regularly rebuild their rigs might prefer this card.
