Finishing with a "Flat Back"

I know what this means but what benefit does it have to finishing like this? I was told it slows my shoulders up to not finish flat…

watch video of nolan ryan after he throws a pitch. vern ruhle (major league pitching coach and teamate of ryan at houston) used to say ryan wanted to visualize balancing a cup of water on his back without spilling it. his head is way down there looking at the plate after he releases the pitch. if you stride farther than your height (which you should strive to do) it’s relatively easy to end up there.