A control maintains its view state in a Control. The State methods allow you to programmatically restore internal state settings between page views. A page experiences something called a postback every once in a while. An example of this is a registration form with multiple text fields that must be resubmitted and validated. Think of a postback as a chance to refresh the page.
Controls have the opportunity to process data during a postback.
That is, a control has the opportunity to process any incoming form data and update its object model and internal state appropriately. This is done so the visual and internal states of the control remain in synch automatically. Postbacks are handled by the IPostBackDataHandler interface. Once a page has been activated, a control needs to know when it's being loaded. A Control may have to notify the client that data has changed.