Earl of Ormonde· Registered
What these chaps are talking about isn't for security purposes. It's basically a leather book cover that you put on your passport, just like the cover you put on paperback novel.I imagine a passport cover which might be attached somehow, for example, by a neck chain like an identity badge or whatever. The reason why the passcover cover is necessary is because without it I don't see how the passport could be attached without such a cover.
The only time I used one was on my old large dark blue cardboard cover UK passport when I was in the RAF, to physically protect it from the elements and travel fatigue!
Nowadays with plastic covers & with the border authorities needing to scan passport covers into quite a narrow gap, a leather cover is both unnecessary & just gets in the way.
That said, I've never seen a cover that is small enough for modern EU style passports.
The black leather covers, specifically designed for the old very dark blue UK passports, were however very snazzy because they had the gold British coat of arms on the front, as did the passport, and two cutouts to reveal in turn the two cutouts on the passport cover containing passport number and name.
Old British passport, the leather cover was exactly the same https://farm4.static.flickr.com/3362/3481508410_c7cbebd8f2.jpg