Events In Yii2

Let’s discuss concept behind events in Yii2. What are they for? I know the purpose of events is use callback functions in certain places. But there are some features in Object Oriented Programming. Because we need modularity, minimal cohesion and other OO Principles. So what are these features?

I see many files in Yii2 with partial name ‘event’: UserEvent.php, ModelEvent.php, ViewEvent.php. May be it is redundant to blow code and there are more simple solutions.