It is used for iTunes to find music shares among different computers via networks. It is also used by iChat to find other iChat users on the LAN.
Without special DNS configuration, Rendezvous only works on a single subnet. There seems to be a misperception that a service made available over Rendezvous is "on the Internet", or available to the world-wide users of the IP network known as the Internet. This is not true. Services are automatically made available to only the users of a single ethernet network, which usually encompasses a fairly small area.
The term "rendezvous" is also known as a synchronization strategy for message exchange of machines and persons.
Programss are not the only ones that synchronize at certain points. An aspect of life is that contacts and lovely datess happen all day.