Each time the user-PI sends a command to the server-PI over the control connection, the server sends back a reply. FTP replies serve three main purposes:
They serve as confirmation that the server received a command.
They tell the user device whether or not the command was accepted, and if an error occurred, what it was.
They communicate various types of information to the user of the session, such as the status of a transfer.
For a human user, a string of reply text would be sufficient to satisfy the requirements just mentioned, and FTP replies do include descriptive text. But having only a text string would make it difficult or impossible for FTP software on the client side to interpret ...