Why do we ned to close listen socket in child process in a concurrent server ?

Hi Experts,
My question is " What happens  if we don't close listen socketin child process?. It is mandatory ?
Generally we write in  following manner.
bind(listensock, (struct sockaddr *) &sAddr, sizeof(sAddr));
Afterward, we put the socket into listening mode to listen for incoming connections.
    listen(listensock, 5);
We then call accept() and allow it to block waiting for connection requests from
 clients. After accept returns, we call fork() to create a new process. If it returns 0, then we are in the child process; otherwise, the PID of the new child is returned.
     while (1) {
      newsock = accept(listensock, NULL, NULL);
      if ((pid = fork()) = = 0) {
     printf("%s\n", buffer);
Once in the child process, we close the listening socket.
We know  that all descriptors are copied from the parent process to the child.
The child process does not need the listening socket any longer, so we close the
child's reference on that socket.
     close(listensock);
     nread = recv(newsock, buffer, 25, 0);
     buffer[nread] = '\0';
     send(newsock, buffer, nread, 0);
     close(newsock);
     exit(0);
    }