Well it’s a subdomain. You could have www.example.com and ftp.example.com go to the same computer, or you could have a load balancer direct www to multiple computers. But it’s helpful for keeping stuff organized in your configuration to give them different names based on what kind of content and/or protocol they’re serving out.

Google does a pretty good job at this where it’s mail.google.com for gmail, www.google.com for their search, and if you’re logging into any google account, it’s something like accounts.google.com. That way you know which company you’re exchanging information with from the domain name, and the subdomain tells you which service you’re using. Helpful if you want to make sure it’s not a phishing site.