The Global Illumination light is a shortcut for when you are lighting an interior with no visibility to the outside environment, and don’t want the renderer to waste time checking visibility of the environment map. For example, a room with no windows. This is only useful if you are using photon maps, since the environment map is not contributing light to the scene.
This tool creates an Environment light and turns off the Add background light parameter and turns on photon map creation.
This tool is not actually a light. It is a photon map generator that requires one or more lights in the scene to see results.