If you work with Ajax you probably encountered situations when it would have been handy if you could render an element directly from the controller. But that was not possible. You had to use a view, and then from there you could render the element with renderElement().
In the meantime this feature has been added to the development branch (I don’t know when) as I learned today from a comment by gwoo.
If you have an element /app/views/elements/example.ctp in your application, then you can render it with the following code snippet from within your controller:
In the same way you can also render views which belong to other controllers.