The difference is the core clock speed of the graphics card. The higher the core clock, the higher performance of the card.
Basically it lets people buy graphics cards that are already overclocked, so you dont need to worry about overclocking them yourself.
not to mention their usually binned cpu's, which will perform better than their normal binned buddy's
alot of manufacturers will give em hyped up coolers as well to improve overclockability past there already set o/c