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

I think they did this so that while a transfer was occuring you could continue to work with the server and start new transfers easily (if your client could support this).

Note that passive mode solves nearly all firewall/NAT problems.