The Telerik RadAjax controls update the ViewState of the page after the AJAX request in order to keep it in a consistent state regardless of the type of the request that has been made (AJAX or postback).

In this example we use the ViewState to store the value of an integer variable. After each AJAX request (postback) the value is incremented by one. If the RadAjaxManager did not keep track of the ViewState, then the value would always remain the same.
This could be seen by simply setting the EnableViewState property of the Page to false by clicking on the checkbox. The EnableViewState property is set true by default, so that Telerik RadAjax controls keep the ViewState consistent.