(Pics:) Can a tailor explain how back curvature impacts the cut of a shirt

I've had five shirts made by ModernTailor. I picked average back/average shoulders for a couple of the shirts and bent back/straight shoulders for the others. Can somebody knowledgable explain how exactly this information impacts the cut of the shirt, because to be honest, after experimenting with this in five shirts I have the impression that it makes f**k-all difference - here's one example of each:

average back+average shoulders

bent back/straight shoulders

No darts. I used to get all my shirts with darts but have grown tired of the look of late.
I assume when you give them finished-shirt measurements darts or no darts has no influence on the fit anyway.

The cuff buttons in the picture of the 2nd shirt are not all buttoned, I noticed now.