Well, it wouldn't be too hard to imagine a scenario where someone needs to SSH into the company WWW server on port 22, which is of course a different machine than the FTP server that they also sometimes need to SSH into on port 22, etc... Naming groups of machines under a subdomain that describes the function they perform is an easy way to reconcile the fact that we sometimes have multiple instances of the same service running on different machines in our network (and need to access them individually).
It makes life easier if I know I can access my web servers at www.whatever.com and my FTP servers at ftp.whatever.com, rather than trying to remember that morpheus.whatever.com runs web services because it has port 80 open, or needing to worry about the FTP server also running an HTTP server on port 80 for its stats dashboard, etc.
