If you want routing to handle all requests, even requests that point to files, you can overwrite the default behavior by setting the RouteExistingFiles property of the RouteCollection object to true. When you set this value to true, all requests that match a defined pattern are handled by routing.
ASP.NET routing enables you to use URLs that do not have to map to specific files in a Web site. Because the URL does not have to map to a file, you can use URLs in a Web application that are descriptive of the user's action and therefore more easily understood by users.