In reply to mark k..

One possible use case would be to provide a front end that is not obviously WordPress powered (so less of a target for hacking attempts), can be secured more fully (place the WordPress backend content generator portion in a more locked down section), doesn't need to hit PHP and a DB to generate a page (less server resource intensive, also well written javascript is amazingly fast now days), the content can be placed into layouts that can move far away from the standard WordPress theme template approaches (layouts that don't have to think in terms of header, main, sidebar and footer), and javascript can allow for more fluid user interactions with data (ajax and instant UX changes).