The File Transfer Protocol (FTP) is a standard network protocol used for the transfer of computer files between a client and server on a computer network.
FTP client for WordPress is built on a client-server model architecture that uses separate control and data connections between the client and the server.
When it is added on a website it helps organize, edit and check. And If there is some problem on the website that needs to be fixed or looked upon.
Not every FTP client is the same. Every FTP client is different from one another. Every FTP client functions differently and has its own pros and cons.