When doing plain compositing objects into pre-rendered or video footage, the objects do not look very integrated, and one big reason for this is that they don't cast shadows. In actual fact, they do cast shadows, but the background just can't receive them. Another problem is that if your background plate has any reflective surfaces, like a shop window, it will look odd because your objects won't be reflected in them.
One of the best ways of adding shadows and reflections to such scenes is to use "shadow catcher" objects, dummy objects in the shape of items in the background that show a background image unless there is a shadow falling on them or a reflection needs to be shown, and this is what catcher does when applied as a reflectance shader.
Click here for a very basic usage example.
You could achieve some interesting effects by setting the background image in this shader to something other than that of the background shader.