In an ASP.NET web application that does not build use of routing, an incoming browser request should map to a physical file. If the file does not exist, we get page not found error.
An ASP.NET web application that does build use of routing makes use of URLs that do not have to map to certain files in a Web site. Because the URL does not have to map to a file, you can use URLs that are explanatory of the user's action and therefore are more easily understood by users