ASP.NET MVC NonAction meaning

Can anybody please tell me why should I use the NonAction attribute? I mean say I have a form with several submit values: Update, Delete or Insert. Since all the submit buttons have the same form in common I'm switching the submit value inside the controller and act accordingly.

Like this:

public ActionResult asd(string submitButton){
     switch(submitButton){
         case "Insert":
             return Insert();
         // bla bla bla
     }
}

[NonAction]
public ActionResult Insert(){
    // some code inside here
    return View();
}

Once again, why should I use NonAction instead of something like this:

public void Insert(){
    // some code inside here
}