> What's the difference of the vast rare books in private collections

There's little difference and I would suggest there's a pity in knowledge being hoarded by collectors.

If we were getting into the differences, though, it would be that is now happening en masse in order for a handful of corporations to monopolise knowledge, history and ideas from books that were until recently available somewhere to the public.

I also doubt that many collectors are buying rare books with the intention of destroying them.