If I understand correctly, the main difference is that the controller no longer calls the view (templating system or what-have-you) to output the result, but rather outputs the model and the responder figures out how to render that data.
I find this useful because it allows you to reuse a controller (action). In most applications you'll have at least 2 views on the system: UI and API. Sometimes, they're the same, but very often they're not.