1. Two commands instead of one. :)

2. Scripting an scp, rsync etc from the internal machine is easier now, since the ssh from the external to internal machine is handled transparently for you.

i've seen this used for security, in that you can have an extra external server you must have access to in order to get to internal servers.

Ah ok, this make sense especially if some of your machines are on the same LAN but don't have an external IP.