iMo, a photograph can accomplish 2 things. It can illustrate a referent and, in the best of cases, it can illuminate, not only the referent but also the totality of what is depicted within the frame imposed upon it by the picture maker. In fact, to illustrate and to illuminate are intimately connected in the same act. That is, to illustrate-in our case to photograqph-is to create an illustration-in our case a photograph-as an example of something (in the broadest sense) which most often is employed to elucidate, aka: illuminate, or prove something about that which is illustrated.
So, it seems very clear to me that people who engage in the act of making photographs are, in fact, illustrators who are creating illustrations of something in order to illuminate the visual characteristics / qualities of that something, most often to make a statement or prove something about that something.
Simple enough, no?
FYI / IN MEMORIUM One of the most influential photographers of the last century-albeit that that influence was based primarily on a single book-is dead. That photographer would Robert Frank and the book would be his book, The Americans. There is a good article in today's NY Times but that article is behind a paywall.