Why does login_tty(int fd) close fd at the end?

[Regid Ichira <regid23@yahoo.com>]

  Isn't closing fd at the end makes all the work meaningless?

    $ man login_tty| grep -A6 'The  login_tty() '
           The  login_tty()  function  prepares for a login on the tty fd
           (which may be a real tty device, or the slave of a  pseudoter-
           minal  as  returned  by  openpty()) by creating a new session,
           making fd the controlling terminal for  the  calling  process,
           setting fd to be the standard input, output, and error streams
           of the current process, and closing fd.