I was doing some work on a hook_setting() page and needed to do some changes to the returned form before it was past to the default standard system_setting_form_submit() function to save the settings.
I didn't really want to duplicate the system_settings_form_submit(), but if I wanted to add my own _submit() hook the standard one would not be called, and the form would not be saved.
Formsapi is very powerful so I just added the following to the form, and it will now call both my function and the standard function.
$form['#submit'] = array(
'example_settings_form_submit' => array(),
'system_settings_form_submit' => array()
Now this will call my function first, and because in the submit() hook passes $form_values by reference, the changes that are made to it are past to the default function.