Because the clients need to get the list of nodes from somewhere.
You can read more about how this works at this URL: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt
You can read more about how this works at this URL: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt
Previously, it also joined an IRC channel and got the list of hostnames from all of the other users in the channel.