“Eye rays” are the primary rays; the other ray types are considered secondary.
One use of this shader is to specify different environments, depending on the type of ray:
Specifying a Reflections shader can speed up render time by sparing the renderer from having to ray-trace the scene.
Another use of the Color Override/Ray Type Switcher can be to eliminate color bleeding in the scene.
Wood grain on floor causes color bleeding on the walls, especially at the right of the scene.
The solution is to use a Color Override/Ray Type Switcher, and wire the diffuse map so it affects Eye Rays only.
Color Override/Ray Type Switcher restricts the effect of a bitmap to Eye Rays only.
When the bitmap affects Eye Rays only, color bleeding is gone.