Private Tor networks: what are the benefits, and how would it be accomplished?

The anonymity provided by Tor stems for the large number of cooperating nodes; a "private Tor network" looks like ultimate counterproductivity. Wait, no, a private Tor with peer authentication is even better in that role: anonymity is so much more guaranteed when you make sure of who you are talking to.

Nevertheless, Tor is free software so you could download the source code, compile it, and start from that point.