Bob has a few pretty defining characteristics:
• He is the only male character from the support groups to have a first and last name.
• He's one of the only characters with a name who dies.
• He has giant man boobs.
When our buddy Bob dies, members of Project Mayhem and fight clubs across the country chant his name. Why? Well, according to Project Mayhem, "Only in death will we have our own names since only in death are we no longer part of the effort. In death we become heroes." (24.26).
But wait, we just said that Bob had a name before Project Mayhem. Ah, irony. Bob gave up his name to be a part of the Project, and only when he died did he get it back. Was the sacrifice worth it?
So how does Bob fit into the novel as a whole? He definitely bridges a couple gaps for our narrator.
(1) The gap between masculine and feminine. Only when snuggled up between Bob's giant breasts can our narrator cry, something he feels is a very feminine act.
(2) The gap between support group and Project Mayhem. It's easy for our narrator to separate Project Mayhem from reality when its only members are a bunch of nameless space monkeys. But when Bob joins, it gets a little more personal. And so, when Bob dies, our narrator finally decides that Project Mayhem must be stopped. Without Bob and his ultimate sacrifice, our narrator might never have woken up to reality.