One reason: DNS suffix search.
In my workplace, I put intranet in the address bar and hit enter, and although it doesn't find a DNS match on intranet, it knows to look for intranet.[my domain] or whatever (these default search domains are pushed out via DHCP or automatically assumed based on own domain name). Same applies to mail, ftp, proxy, etc, etc.
It's sort of a way to do private addressing for hostnames - if I see an unqualified name, it's always assumed to be in the same domain that I'm in or something fairly local. This can be a very handy shortcut, and is very widely used in private networks. I think it makes a lot of sense, but obviously this wouldn't work if intranet was a valid FQDN in its own right.
Good day to avoid cops. Crawl to work.