A Control's Before Control event is triggered when its
Page is displayed for the first time. Before Control events are triggered for each Control, in the order they appear in the Page. These events immediately follow the Page's Before Page event.
A good use case is when the Control is part of a Component. The Control's Before Control event will be executed in the context of the Component, and not the Page the Component has been added to. This means that the Control's Before Control event will work the same way regardless of the Page it has been added to.
In most other cases we would recommend that you attach your code to a Before Form or Before Page event instead to make your application easier to maintain.