[PHP] Question about session_id() and session_start()

I find the Note in PHP document.
http://www.php.net/manual/en/function.session-id.php

"Note: When using session cookies, specifying an id for session_id() will
always send a new cookie when session_start() is called, regardless if the
current session id is identical to the one being set."

I feel puzzled about this feature. Even if the current session id is
identical to the one one being set, session_start will send a new cookie. I
want to know why session_start behave in this way.

Forgive my poor English. Thanks in advance.