The only real reason for self-referencing is the fun factor. It's fun for the writer, getting little peeks at what old characters might be up to. And it's fun for readers to spot a familiar face, or pick up on a made-up book title or something from an earlier story. I don't know that it does -- or even should -- contribute to the story in hand being any better than it would have been without it.
Of what use were it, pray, to get a little wood to burn, to warm your body this cold weather, if there were not a divine fire kindled at the same time to warm your spirit?