Backbone.js gives structure to web applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing API over a RESTful JSON interface. Backbone simplifies complex JavaScript applications by providing a clear separation of concerns between data, presentation, and user interaction, making code more organized and maintainable. It's lightweight, versatile, and a great choice for building single-page applications.