The information above about this function that it can only be used BEFORE session_start depends on how you use it. Because its also useful AFTER a session has started as follows:
Example you wand to change an already set value of the session cookie expire time:
<?php
// Here we start as usual
session_set_cookie_params('3600'); // 1 hour
session_start();
// More code...
// Now we found in some database that the user whishes
// the cookie to expire after for example 10 minutes
// we can change it instantly !
session_set_cookie_params('600'); // 10 minutes.
session_regenerate_id(true);
// This will delete old cookie and adopt new expire settings and the
// old cookie variables in a new cookie
?>
Please note i only explained the browser (client) side changes of session cookie's expire time.