Originally, the FTP protocol connected back from the server to a client to actually transfer files through that new connections. 14 years later after the introduction of the FTP, the 'passive mode' was added to it, so that only client connects to server ever, however a need for multiple connections remained.
Compare that to HTTP or SSH protocols, where many things could be done through a single connection.
The NFS protocol (until the late NFSv4) has a rather chatty connection process, where the client first connects to the 'portmapper' (at a fixed port 111) to only get a port of the 'mountd' daemon, then connects to that daemon only to inform the server of a mount a client is about to do. After that, client returns to a 'portmapper' to get regular NFS port (2049), and proceed there with regular operations. The same kind of chatter happens when client wishes to use additional services like 'rquotad', 'lockd' or 'statd' -- each at its own port.
The funnier thing is that all but 'portmapper' (111) and regular NFS (2049) ports are also dynamic.
Then, in NFSv4 everything at last collapsed back into a single port (2049).
So my questions follows:
What were the design rationale behind not putting the whole protocol into a single connection, at a time those protocols were designed? What benefits emerged specifically from doing server-to-client back-connections in FTP?
Overall, what benefits got the designers planning the protocols in such peculiar ways?