Hi Green. Besides the things teor mentioned the DirPort also provides easier access to directory information which Stem's remote module...
https://stem.torproject.org/api/descriptor/remote.html
... and curl can take advantage of...
% curl 128.31.0.34:9131/tor/server/all
- relays no longer fetch documents using the DirPort (so maybe never).
Hope we can amend this to be 'relays *and* stem', otherwise DocTor and quite a few things will be very sad. :P
Cheers! -Damian