The envirosphere casts light (you can render a scene with no lights and just the envirosphere and you'll have a render). The light is completely ambient but your scene will still be lit. It also gives your reflective surfaces something to reflect. For instance, put an image of the sky as the texture on the sphere and you'll see the sky and clouds reflecting in your windows, chrome car bumpers, whatever. You can also use a light with it to simulate the sun.