You can use CI as a backend, no doubt about that. Its a full fledged MVC framework. But as it is a MVC framework, the controllers used to return views where you display data from controllers. If you are using CI just as a backend, then you have to just return data in json format from your controller instead of a view.
70.0%United States of America (the)
14.6%Russian Federation (the)
10.2%China