RouteTable.Routes RouteCollection?

The example I see everywhere for MVC-style routing is something like this:

void Application_Start(object sender, EventArgs e) 
{
    RegisterRoutes(RouteTable.Routes);
}

public static void RegisterRoutes(RouteCollection routes)
{
    routes.Add(new Route
    (
         "Category/{action}/{categoryName}"
         , new CategoryRouteHandler()
    ));
}

What's the reason to pass the RouteTable.Routes collection to RegisterRoutes()? Why not just:

void Application_Start(object sender, EventArgs e) 
{
    RegisterRoutes();
}

public static void RegisterRoutes()
{
    RouteTable.Routes.Add(new Route
    (
         "Category/{action}/{categoryName}"
         , new CategoryRouteHandler()
    ));
}

What RouteCollection besides RouteTable.Routes would a route be added to? Isn't RouteTable.Routes the RouteCollection for the web application?

I have a particular IRouteHandler with has a Map() method:

public class ChatRouteHandler : IRouteHandler
{
    private static bool mapped;

    public void Map()
    {
        if (!ChatRouteHandler.mapped)
        {
            RouteTable.Routes.Add
            (
                new Route("chat/{room}/{date}", 
                new ChatRouteHandler())
            );
        }
    }

Is there a reason that Map() should accept a RouteCollection and not add to the RouteTable.Routes collection? Again, what other RouteCollection would this IRouteHandler be added to?