1) In normal ASP.NET, the physical file location must be match with the URL. If file not present then it will give error 404 page not found.
By using Routing their is no need to file physically present to that location, if file not present in directory then browser searches in whole project directory.
2) By using routing, if we call to the only controller then it automatically called to particular Index related to that controller.
3) Default ASP.NET MVC routes is defined in the Global.asax file, as
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { controller = "Home", action = "Index", id = "" } // Parameter defaults
);