[drupal-devel] What does #post_process do?

In http://drupal.org/node/35575 Karoly suggested that the form needs a #post_process. The only documentation I can find about #post_process is "This also introduces the #post_process property, which allows you to change the form based on stuff that has been submitted." Well, this bug doesn't require the form to be changed. It just needs to read a field from the form. So I don't see why or how #post_process can be used here. Anyone care to share some wisdom? ...Richard.