A light projector enables you to reproduce and enlarge any image. It's vital for any large-scale artwork - including creating murals, as you can simply project the outlines onto the wall or canvas and trace over them. It's also incredibly useful when you want to enlarge any image, or to turn a 3D object into a 2D outline.
Light boxes are incredibly useful whenever you want to trace a design, whether you simply want to reproduce a motif for scrapbooking or other papercraft, or transfer it onto another medium, for example for embroidery patterns or outlines for fabric painting.