There's shitloads of reasons for alts. For instance a member that owns a business would want to do official business related stuff via an alt just for that purpose and have a regular account for talking freely. Lots of people here and elsewhere do that. (Of course Josh/Inaba fucked that whole thing up so royally so it kinda has some undeserved bad reputation)
Bitcoin100 and Phinnaeus Gage is the perfect example of this. Bruno screws around constantly as Phinnaeus Gage but when he used to post as Bitcoin100 he was always professional - well, professional for him.
I've had some great fun along time ago screwing with Matthew by arguing with him and then turning around and siding with him as someone else. Then at the last minute I would get both accounts revved up against him. LOL