It's very useful for cooperatively pretending that a single machine is actually a network of machines, without all the madness of network namespaces.

Basically every server program ever supports the notion of "which IP address am I supposed to bind to?" Normally the answer is either "localhost" or "whatever my LAN or WAN address is, for a particular interface". And specifying this (both for servers and for clients) is much less effort than replacing all the standard-ish ports with ad-hoc ones.