OMG, all you people with "theories" on how or why you would shoot medium format is just hilarious. "Can't shoot wide open because depth of field is paper thin." Uh, no it isn't. It's usually f/2.8 or f/4. Is that usually paper thin in full format? No. Well, it isn't in medium format either.
"Medium format is for product and still lifes." Again, nope. It's for whatever makes you happy.
Instead of guessing and spreading false information, why not go out and actually buy and use it? It's not that expensive and for the price of a D850 body you can buy a pretty good Hasselblad outfit and definitely a lot of other brand products.
As for why all the medium formats? Because the photographic industry is hurting for sales and the next big thing (so to speak) is medium format. They want sales. They want to sell you new lenses and new bodies. I don't think the 44x33 sensors give a whole lot of difference between full frame, but that's not stopping people from buying it.
All the photos below are 85% quality and reduced down further than necessary so they may not look sharp, but they are.
No studio here:
Not too paper thin:
and some other examples:
https://www.flickr.com/photos/nathantw/albums/72157681384071001/with/34369569321/