A resposta pode ser encontrada em este artigo da wikipedia:
Applications implementing common services often use specifically reserved, well-known port numbers for receiving service requests from client hosts. This process is known as listening and involves the receipt of a request on the well-known port and reestablishing one-to-one server-client communications on another private port, so that other clients may also contact the well-known service port.
Além disso, do artigo sobre sockets de Berkeley na wikipedia
When an application is listening for stream-oriented connections from other hosts, it is notified of such events (cf. select() function) and must initialize the connection using the accept() function. Accept() creates a new socket for each connection and removes the connection from the listen queue.