Rendezvous is based on the "ZeroConf" open-source project, and is designed to allow not only automatic discovery of network services, but also very advanced file-sharing and P2P features as well.
Basically, it means you can get your network going without having to know anything about IP networking.
The other main advantage is that it allows developers to use one common protocol - TCP/IP - for all their network apps and games. Until now, games on PC needed to have TCP/IP for internet games, and IPX for setting up quick local networks. For the mac, it was TCP/IP and AppleTalk respectively. This meant the apps would talk to each other in the common format, but there was no quick 'n' dirty way to get a mac talking to a PC. That, and the fact that AppleTalk is on its way out of favour.
That means a system that automatically establishes an IP network is very, very useful and allows machines to talk to each other quickly and easily in a common format. And now developers need only support IP in their software.