Enable ViewState turns on the automatic state management 
feature that enables server controls to re-populate their 
values on a round trip without requiring you to write any 
code. 

This feature is not free however, since the state of a 
control is passed to and from the server in a hidden form 
field. 

You should be aware of when ViewState is helping you and 
when it is not. For example, if you are binding a control 
to data on every round trip, then you do not need the 
control to maintain it's view state.

ViewState is enabled for all server controls by default. To 
disable it, set the EnableViewState property of the control 
to false.