One of the advantages of using your own parent controller is that the common checks can be checked at central position. so when you are creating/defining a controller, you can inherit from your own controller, like below to use parent class methods:
class User_Controller extends MY_Controller
{
public function login()
{
if($this->is_post()){
//login check and redirect if successfull.
}
$this->load->view("login");
}
}