Even though photorealistic rendering is very popular in modern computer graphics, non-photorealistic visual styles are also popular.
Especially since the inclusion of the Freestyle render engine with Blender 2.67 and onward. Commonly referred to as “NPR”, this style of rendering can be generalized as “cartoony”. As you can imagine this is useful for visual styles like anime, commercials, architectural visualization, and motion graphics like the ending credits of “Tears of Steel”.
Freestyle’s purpose is to detect geometric edges and draw stylistic strokes over them. Artistically, we can think of this as “outlining” our objects. And when these outlines are combined with toon-style shading, it’s very possible to achieve hand-drawn-like treatments of our 3D scenes. We can also render Freestyle lines by themselves to achieve a look like technical-drawings.