Why do you put a pen or a geological hammer in photographic geology?

It's a reference. In order to give the viewer an idea of how far the picture is from the actual scale, it is equivalent to putting a ruler. Just like you took a picture of the earth and table tennis. No one knows how big the earth is, but table tennis players all know how big the earth is together.

Geologically speaking, whether you take a picture of a mountain or a rock, the geological hammer is a reference.