when to use parent::__construct(); in the class?

[eluser]Unknown[/eluser]
Dear all
I`ve started learning CI by the tutorial in the User Guide.
I found the News_model did not add the parent::__construct()

Code:

News_model extends CI_Model {
public function __construct()
...//

But the News controller starts with parent::__construct()

Code:

class News extends CI_Controller {

public function __construct()
{
  parent::__construct();

Could anyone kindly tell me the differences?
And whats the criteria of adding the parent::__construct() when creating the class?
Many thanks