I guess the recognition I get from fellow gamers, good and bad, is a big part in why I multibox.
It's just fun when people ask questions or just want to see some mayhem being caused and lurks silently behind my characters to see some kind of spectacle.

I also do it for the added skill that requires to play more than one character, it certainly made world of warcraft more appealing when I would have quit if I single boxed long ago.

...and ofc grouping with myself and being able to gank people by myself.