Let's say you're writing an app like Google Docs, which does a lot of work on the client.

Before long you're going to have a real mess of Javascript variables, DOM elements and arrays of stuff, which is why it's useful to have MVC structure on the client as well as the server.

Your server models sync automatically to your client models, you can hit model.save() to persist your client models, and your client views represent the data in a way which can be quickly adapted if requirements change.

For a less dynamic app you don't need it.