In protocol design, why would you ever use 2 ports?

Question

When a TCP Server does a socket accept on a port, it gets a new socket to work with that Client.
The accepting socket remains valid for that port and can accept further

Answer 1

FTP has a very long history, being one of the first ARPANET protocols back in the early seventies (for instance, see RFC114 from 1971). Design decisions which may seem odd now made more sense then. Connections were much slower, and performing the connection control "out of band" probably seemed a good move with the available network technology.

The current RFC959 includes a good section on history, linking to earlier RFCs, if you fancy doing a little archaeology...