The problem with passing in your routes when you create a Router is that you have to supply all the routing functions together with your routes, which (especially if you have a lot of routes on your site) can easily become messy. Instead, many Backbone programmers define their routes on the Router class itself, as shown here:

var SiteRouter = Backbone.Router.extend({
    routes: {
        'foo': 'fooRoute'
    },
    fooRoute: function() {
        // logic for the "/foo" or "#foo" route would go here }
});
var siteRouter = new SiteRouter();
Backbone.History.start(); // siteRouter won't work without this

As you can see, this approach simplifies the routes' definition and makes them more similar to the events properties of Models and Collections. Instead of defining routing methods with the routes themselves, you can simply give the route the name of a routing method and Backbone will look for that method inside the Router class.

There is also a third way to add routes and that...