As far as I'm aware, it's all about your measurements... like if you're exceptionally tall, or if you have really long legs coupled with a short torso and short arms (short reach). Things like that can be reason for a custom frame, although IMO just about anyone can find a frame out there that'll work for them off the shelf if they look hard enough.
EDIT: Oh yeah, and for cosmetic purposes too, like paint.
Last edited by grahny; 05-05-08 at 05:07 PM.