In Yii,why and how to use Yii::app()->end() method?

In form validating,I find such codes

if(isset($_POST['ajax']) && $_POST['ajax']==='login-form')
     {
         echo CActiveForm::validate($model);
         Yii::app()->end();
     }
 

The manual says that the end() method will terminate the application.
Why to terminate the app? The following codes will not execute?

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