Just before the end boss, Noctis wants to select a photo from Prompto, to take with him.
So you have, as choices, all photos that you made Prompto save during the trip.
But, if you never saved a photo, or deleted all saved photos before that point, what are your choices?
EDIT: I don't know how my question is not clear. I'm not asking what do photos do, but what happens in the big moment at the end where you have to choose one of your saved photos. If there are no choices, what happens? Does it just skip that part of the cutscene, is it prefilled with photos that Prompto will say 'oh, we dont have any, lucky us I got a secret stash!'