@CharismaticBatman @royal In the case of the National Library of Scotland, it's because they are a legal deposit library so if it was published in Scotland, they have a copy. A big chunk of their books are one of a few left in the world. Borrowing is forbidden but, if they happen to not have a book, they have ways of getting a copy.
I do love stack browsing too though. Looking forward to my uni library letting alumni back in.