There are cases that model validators should be depended by controller/action and you couldn't manipulate on the model class or using scenarios in easy way

Some times the core validation rules provided by Yii won't satisfy all your needs, so you'll need to create your very own validation rule.

This is a reference to be used for Model rule validation and is compiled from the Yii documentation and code. The purpose is to have all the information gathered in one place instead of scattered. This reference is not an intro. See The Definitive Guide to Yii, Declaring Validation Rules for a tutorial.