The fill light helps light up areas of the image that the key light does not illuminate. Check out what the fill light can illuminate by itself:
Select Layer 2, and then press the Shift key while selecting Layer 4.
Press F12. The image should look similar to the following image. Press Esc when you have finished looking at the image.
Now you see only the fill light, which is to the right of the camera, as seen on the right side of the following screenshot. The light is dimmer, about one half to one quarter the value of the key light. It is also lower, so you see the highlights of the fill light come off the sides of her head and body, as you can see on the right side of the following screenshot. The purpose of the fill light is to fill in the areas that are in shadows created by the key light. You can see that the fill light will give important details about Sintel's left ear and the shape of her neck.
