warning: Parameter 2 to ad_form() expected to be a reference, value given

I need to load some fields from ad_form() and got this error (warning: Parameter 2 to ad_form() expected to be a reference, value given) when i call drupal_retrieve_form('ad_form',$ad_node,$form_state) it returns empty. To solve problem i change ad_form(&$node,&$form_state) to ad_form(&$node,$form_state). Why You use value reference at all?