What is the purpose of ports on a server?

An IP address finds the right machine; the port number then picks which application on that machine should handle the connection.

One server (one IP address) usually runs several network services at once — a web server, a mail server, a database. The IP address alone cannot say which of them an incoming connection is meant for. Ports solve this: each service listens on its own numbered port, so the address-plus-port pair routes the request to the right program.

Picture one host server.example.com running several services:

Many protocols have a well-known standard port so clients know where to knock without being told:

Tip: because the standard port is implied, http://example.com quietly means http://example.com:80. You only spell out the port for non-standard setups like a dev server on :3000.