JavaScript MVC frameworks like Backbone.js are for adding structure to your front-end.
This is most useful when building [increasingly popular] single-page JavaScript apps (SPJA). If you're heavily using unobtrusive JavaScript, you're probably doing a fair amount of ajax for dynamic content to avoid refreshing the page on the user. SPJA's take this a step further by letting the user visit all areas of an app without ever refreshing the page. Without the structure provided by MVC frameworks, the client-side code can quickly get out of control.
