Difference between "MapHttpRoute" and "MapRoute"?

Why using "MapRoute" for "Default" routing, while using "MapHttpRoute" for "DefaultApi" routing?

routes.MapHttpRoute(
  name: "DefaultApi",
  routeTemplate: "api/{controller}/{action}"
);

routes.MapRoute(
  name: "Default",
  url: "{controller}/{action}/{id}",
  defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);