The problem is everywhere for a typical multi-tier architecture. The UI calculation is done at server-side and the browser just displays the data. That was the typical architecture for several years. Every interaction with the user needs the server backend. Especially with lower bandwidth this is a big issue and the server needs to know the client state, which cases additional memory and processing capacity. And on top the user interface feels sluggish. Increasing the server capacity doesn’t really help in the most cases…
By using the approach you get a solid fundament to delivery real business applications for mobile devices. These hybrid applications can run with the same implementation on several platforms and also leverage some platform features, like push notification or background network fetch.
Such a sample application which uses a RESTful backend and WebSockets was made by me for demonstration purposes. You can download it on GitHub. The sample shows that HTML5-based hybrid apps with a native container like Apache Cordova can delivery high quality apps that feel like native apps in most cases.