The only thing a camera is not capturing is any 3D details and light reflectivity. Unless you are trying to create an absolute perfect replica, I’m not sure why you’d need that level of detail. As an artist you should be able to add that detail yourself.
There's plenty of reasons to not use a photo or a scan, but some trivial reasons are: translucent, metallic/pearl, and fluorescent paints and pigments.
Other reasons include gamut limitations, interactions with light (as you've noted already), and the point of the exercise is to capture minor details and attempt to understand the artist's original decision making processes. That can involve looking at the artwork under different lighting conditions, from different angles and so on.
Photos of course make good reference pieces, but snapping a photo or downloading a scan of a piece is going to leave many frustrating questions unanswered.
Despite the outcome the goal isn't to reproduce the piece. It's to emulate the process that led to the piece.
There are a million reasons why a photograph is not equivalent to an in-person artwork, but the most convincing one in my experience is scale. Computers make every painting into the size of your screen. Seeing a gigantic painting in person by an artist that specializes in scale is an entirely different experience.
As examples: Botticelli’s Venus is a much different experience in person, when you see it from across the room. Anslem Kiefer’s paintings don’t hit you nearly as hard via photographs vs. in person. And so on.
I think the process also involves focusing on and appreciating the craft of the painting through the process of redrawing it.
Otherwise that art-school lesson is rather that every impression in the world can be equally represented by a JPEG on some screen (and while painting is a fun exercise, it's just "a imprecise JPEG with extra steps")...
Even the best photography does a very poor job of capturing what many works of art actually look like. This is an area where tech is not the answer, and those that insist it is are just ignorant of the subtleties of actual artwork. BTW, this isn't some woo-woo claim, there are myriad things that cameras capture very poorly, if at all, starting with the obvious things like reflections (amny types), refractions (ditto), transparency and translucency (esp. in marble!), and many, many more.
