Very often one would take a great photo of a scene. When a viewer wants to know how big it is, the sense of scale is lacking. There is no idea of how big or small the rock, sculpture, building, etc. is.
The idea of placing a common object or people, pens, keys, cars, etc. in the picture helps the viewer judge for himself how big the scene is.
Here Joe McNally goes one step further on how to light up the scene to present a sense of scale and depth of a scene.