By placing lines at various distances from one another, the image gets more structure and gives the illusion of a body. A variation in width gives more dimension to the image and illustrates a sense of realism.
How To : Adobe Illustrator Geometric Art. This is a very interesting type of art. I do like how it uses simple shapes to create a portrait. I know for a fact that you can do this easily in Photoshop with one of the brush tools.