As you'll see if you go to the "Facts" section, Confederacy of Dunces includes a lot of allusions—we're talking historical, literary, pop culture, places in New Orleans, movies, advertisements, and medieval saints. A lot of allusions. Cataloguing them all made our hands cramp, and made us despair of life. So pity us, please; we can feel our valves closing.
Ahem. Forgive our melodramatic outburst; Ignatius must be rubbing off on us a bit. As we were saying, this book is brimming with allusions. But why, you ask? Time to brush the Ignatius dirt off our shoulders, we suppose, roll up our sleeves, and actually get to work on this symbol.
Shout outs serve a bunch of purposes in A Confederacy of Dunces. First of all, they often work as in-jokes of one sort or another. For instance, Ignatius mentions Mark Twain several times, always negatively, like when he says:
"Veneration of Mark Twain is one of the roots of our current intellectual stalemate." (2.185)
This is almost surely a tongue-in-cheek acknowledgement of Toole's own debt to Twain—an author whom, like Toole, loved colorful characters, humorous sketches, and dialect.
On another tack, Dorian Greene's conversation at some points is an almost constant stream of witty allusions. This is funny in itself, but it also is meant to be campy and place him as part of a flamboyantly cultured (and stereotypical) New Orleans homosexual community.
Perhaps most importantly, though the constant references to things outside the novel makes the novel itself seem more real. A Confederacy of Dunces is a world where Clearasil and Plato and Eleanor Roosevelt exist side-by-side, where you can read Boethius or about Batman, and listen to Scarlatti or "Turkey in the Straw." It's a world that, like Ignatius, seems to be bursting out of itself, overflowing its own edges with its sheer quantity of stuff.