They were in close proximity to other books that you knew to be useful. Example: I might be looking at someone's obscure technical book from the 1870s at the college they worked for ... and nearby might be the world's only remaining copy of a bio written by a friend or colleague that mentions facts found nowhere else.
I won't know ... because neither is shelved any more. The 1870s book is found at archive.org. But not the bio. And the computer doesn't duplicate the proximity once found in the physical card catalog.
The rewards of the physical browsing experience are lost in the digital age. No way to assess the damage that's been done by hiding college stacks underground in containers. To assess the effect of moving uncommon books to special collections with infrequent hours and stringent requirements.
I have to wonder about the professionalism of the people who too carelessly make the decision to overturn systems evolved by centuries of scholarship.