How many times have you heard " That photo doesn't do it justice"
Or when you see a photo of a big trophy fish it has a ruler next to it.
Sometimes when you take a photo of an object you just can't tell how big it is without something added for scale.
A good example is this photo of a Giant Sequoia tree in California.
I took this on a recent trip to the west coast. At first I took some of the trees but when I viewed them I really couldn't see the mass and size of these monsters of the forest.
But adding a model makes you look at the tree and then when your eyes fall down to her hugging it you then see the comparison in size to a person.
Besides the fact that I do think most photos benefit by having a person in them.