It almost looks like after a time some of those wealthy enough must have seen the triple portraits and said, "I want one of me too!" even though unrelated to sculpture.
Of course some of those triples might also have had other purposes such as trying to test some angles, lighting or such before a final work.
But it does seem that as time went on the practicality and purpose was lost giving way to exercises in artistic invention.
The practical use for sculpting certainly makes more sense to me and I must assume sculptors rarely cared to share their preproduction paintings as the sculpt was the entire priority.
Also with the advent of photography I can see where the references could be much more plentiful and perhaps an final photographic montage presented to the subject of the sculpt would only consist of what was deemed the best of the poses which would take away from their usefulness strictly as reference for the sculpt from front, side and 3/4 views.
A lot of conjecture on my part to be sure but regardless its fun to see that once again the more things change the more they remain the same.
Now I find myself a little surprised that 3D artists haven't borrow more from old world sculpting practice although they certainly operating under many constraints when when decaling first appeared as a viable texturing approach.