Sense of Scale
A cue within the frame that tells the viewer how large the subject actually is.
In depth
Without a reference, a photograph gives no absolute size information — a rock face could be three metres or three hundred. A human figure is the most effective reference because its size is universally known, and its smallness in a frame is what converts a picture of a mountain into a picture about a mountain's size.
Examples in practice
A single climber on a cliff face turns an abstract wall of rock into a measurable, comprehensible scale.