The challenge.
To have more to do in WoW while still not needing as much time (ie I can do my daily heroic on 5 characters at once, rather than try and maintain 2 geared alts and need double the time if I single boxed).
It's great fun.
I can AFK anytime I want and nobody gets mad. (I have a family, kids).
To look at old raids/content/rep farm whatever I choose to do more efficiently.
TO make a real difference in battlegrounds/WG rather than get frustrated at the randomness of other people.

I do not solo mbox though. I regularly raid on my 'main', my Paladin and still enjoy all the social aspects of the game just not the 5 man ones.