Might be wrong, but since the manufacture takes the cooling into their own hands, they become more confident in overclocking and are able to use it as a value ad, though most things they do are achievable with most cards yourself. Performance wise, the overclocking ability of a card is in much ways luck of the draw, as with CPUs. Some "extreme" cards will be "binned", or sorted through, to guarantee a certain level of overclocking performance.
EDIT: the addition of custom power delivery is probably a big deal too