The human shape is perhaps the most unmistakable shape in all the world. As a result, when you include its shape in any rendering, an obvious sense of scope and scale result. This happens regardless of whether that rendering is a nature scene, or something urban or industrial.
One of my favorite resources for people cutouts is, by far, VizPeople. I have many of their libraries and they have created a great video on how to composite people in your rendering.