For a wider field of view in most games (more of the game world visible at once), lower GPU horsepower requirements, correct display of the majority of movies (which are designed for cinema screens with similar aspect ratios to this) and perhaps just liking to have that bit of extra width. I've got an article on my website entitled 'The 21:9 (2560 x 1080) Experience' that goes through these advantages.
you will need atleast one 7950 or equivalent if you don't want to sacrifice visual quality to get smooth framerates if you want to run it at 75Hz and get >60fps as much as possible then a 2nd one wouldn't go amiss but isn't essential.