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

When a user starts a FTP session with a remote host, FTP first sets up a control TCP connection on server port number 21. The client side of FTP sends the user identification and password over this control connection. The client side of FTP also sends, over the control connection, commands to change the remote directory.

When the user requests a file transfer (either to, or from, the remote host), FTP opens a TCP data connection on server port number 20. FTP sends exactly one file over the data connection and then closes the data connection. If, during the same session, the user wants to transfer another file, FTP opens another data TCP connection. Thus, with FTP, the control connection remains open throughout the duration of the user session, but a new data connection is created for each file transferred within a session (i.e., the data connections are non-persistent).