So the few POSP type Russian scopes I have owned I always ditched the rubber boot. I wear glasses/would have shooting glasses on so it was a PITA.

Was it simply to provide a military shooter a very quick way to align their eye at the optimal eye box position??

I have always wonder this.

By “rubber boot,” I assume you’re referring to the piece around the eyepiece that extends to the rear—?

Answer: “Tradition; tradition!”

Depending upon the sight and its use, that may be true of some scopesights. As a practical matter, though, that device can, in theory at least, provide a couple of advantages.

One is that it prevents the shooter from “crawling up” on the scope and risking damage to eye or forehead when the rifle recoils. And if he does, the rubber would help minimize the effects.

The second I can think of is to shade the ocular lens from stray light and improve the image of the target.

It’s impossible some times to understand why designers come up with various features and devices, but trying to force the shooter to assume the proper head position to use the sight seems an unlikely reason. If anything, I believe that such devices would tend to make acquiring a target somewhat slower.

I tried such an eyepiece device on a scope one time and promptly relegated it to my spare-stuff-that-will-probably-never-be-used-again box. I found it uncomfortable to use, but although I may not have been doing it right, I couldn’t see any value.

Added: I’m hardly an authority on military optical sights, but I believe one of the earliest that had a rubber eyepiece shield/cover was the Warner & Swasey that was used by U.S. snipers in World War I:
https://simpsonltd.com/warner-...ic-sight-model-1913/

My guess is that the eye shield on the W&S was mostly intended to improve the seeing through the sight. As I understand it, the sight was made in two versions, first with 6× magnification, and later 5.2×. Either was fairly high power for scopes then, and coupled with the size of the objective lens and general optical quality of the day, I suspect that the view was dim by modern standards. The eye shield may have helped with that.