It's got to do with the contention between content and form. Invariably that's what's responsible for its energies, its tensions, its being interesting or not.
What you photograph is responsible for how a photograph looks - the form, the design, whatever word you want to use.
You've got a number of things that take place that are peculiar to still photography. One: how a picture looks - what you photograph is responsible for how a photograph looks. In other words, it's responsible for the form.