It enables the viewstate on the page. It allows the page to
save the users input on a form

It allows to Store Value in viewstate and Persist the 
values across postbacks

when ever your page view state on it will take too much 
responese time. that is why it is better to off the view 
state if you do not require the controls to remeber its 
previous state.

When a form is submitted in ASP .NET, the form reappears in
the browser window together with all form values. How come?
This is because ASP .NET maintains your ViewState. The
ViewState indicates the status of the page when submitted to
the server. The status is defined through a hidden field
placed on each page with a <form runat="server"> control. 
................
Maintaining the ViewState is the default setting for ASP.NET
Web Forms. If you want to NOT maintain the ViewState,
include the directive <%@ Page EnableViewState="false" %> at
the top of an .aspx page or add the attribute
EnableViewState="false" to any control.