Control State (or ControlState) was introduced with ASP.NET 2.0 as an alternative to View State (or ViewState) as a method for preserving the state of a control.
Control State is designed only to store a control’s essential data (like a dropdown list’s selected item) that needs to be available on a page postback. Unlike View State, Control State cannot be disabled. to enable the control to function even when view state has been disabled. ASP.NET stores the control state in for pages in the same hidden element which stores the view state. So if all you wish to do is preserve the control’s basic state over postbacks you can disable View State and Control State will handle all the preserving of state and reduce the page load.