Activating the Outline overlay feature in Photograph setup will make the Photo Lab render the same image twice, using two different techniques (the render mode selected and Contour image). These images are needed to use the post processing effect, also named Outline overlay, which is achieved by placing the contour image on top of the regular image. With the Outline overlay post processing effect, you can control the opacity and thickness of the contour image overlying the regular image. This can be useful in many different situations, for example when drawing a white kitchen where you wish to reinforce the outlines, as in this example:

White kitchen without the outline overlay effect.	Same kitchen with the outline overlay effect. Notice how the floor tiles and arched wall opening remain unaffected.