What is the purpose of EnableEventValidation and how does it work?

Answer 1

There is a good article about how and why you would want to use EventValidation here:

http://odetocode.com/blogs/scott/archive/2006/03/21/asp-net-event-validation-and-invalid-callback-or-postback-argument-again.aspx

and MSDN:

http://msdn.microsoft.com/en-us/library/system.web.ui.page.enableeventvalidation.aspx

The short answer is event validation is designed to protect the web site from having values injected into the page that can be used to exploit your application in some way.