Why use is camera ray in the world

Why do we use is camera ray in the world material:


Sometimes the environment map is of limited resolution, especially apparent when using a camera with a large focal length. In these cases, using a background map for camera only is better. It has greater resolution but often only jpg (not HDRI). Others like to do weird stuff to HDRI without having it affect what the camera sees, but I don’t like this myself.

In some instances, you want a colored environmental light, but a black background.

Making the background black, but only from the camera’s perspective is one way of doing that.

It can be situationaly useful when you want to turn off most of a node network when it is not in direct view of the camera. Although a few more bells and whistles are needed to get it so you get true reflections.
I’ve toyed around with it some and it can be useful on hair/grass simulations, Or anything where you have a scene that is mostly not in view of the camera.
Keep in mind if you use it. Use it with caution. Just because it is not in view of the camera does not mean it does not add to the scene.
In the blend I’m posting you can see the green coming though the ear, Even though it is not in direct view of the camera.

Its also useful in setting up mesh lights that are invisible to the camera. But you need a few other tweeks for that.
http://www.pasteall.org/blend/37822