The system was to keep track of who had borrowed a book, so that they could be tracked down if it was not returned.
I remember it being really fun in grade school, to realize I had checked out the same book as an older friend.
I would be surprised if you are from somewhere more metropolitan than Kobe Japan [0]. It is much more likely that you are just young.
My library card was a 5x5cm card sleeve. If I borrowed something, the return date was stamped in the book, and on the book's removable 5x15cm slip. That slip was put inside my card, and probably ordered by due date in the librarian's index file.
This was early 1990s in Britain.
https://en.wikipedia.org/wiki/File:Library_card.jpg
The Browne Issue System appears to be what most small libraries used.
> The cards were apparently leaked to the paper after a volunteer who once taught at the school came across them while he was sorting out old library books that were to be thrown away.
The reason why we don't hear about these kinds of data leakages is because paper usually gets thrown away. Or a building burns down/floods. And also, paper records don't come up in a Google search (which is the extent of "research" for many people today).
Why didn't the school back then have a policy of throwing away cards after x number of years? Probably for the same reason why anything was done before computers became so ubiquitous...we constantly underestimate how the power of computation and networking changes the accessibility of information. Back when I graduated from college (and by this time, Google was a major player), no one really complained about printing people's names (i.e. mostly students) in the police blotter. Now, I believe it is a rarity for a school newspaper that publishes online to also publish full names of the suspects, even though no federal/state laws have been passed that would require such discretion. The reason why this practice is now rare is pretty common-sensical to us today, but not so much when Google was still relatively new.
It takes up a lot of space, and has to be kept dry within a reasonable temperature range. Academic libraries spend tidy sums on climate control to keep their books from rotting away. It also has to be indexed and filed so it's possible to find what you're looking for.
If you do it properly, you end up with something like this:
http://www.nationalarchives.gov.uk/
So the costs are non-trivial. And most people would consider library loan records trivial information. So there are good economic and practical reasons for throwing them out.
Besides, it's unlikely Plato, Jefferson, or Tesla did most of their reading in libraries. (Karl Marx did, but he was probably an exception.)
Heck, many people keep personal diaries, and most surely would object to people reading them without their permission while they are alive. On the other hand, their diaries are prime sources on given person's life to biographers, which is one reason some people keep them.
I understand literature and history departments at universities love this kind of sources.
You may be thinking of the courtesy extended to the living heirs of dead people.
"Carmichael’s words continue to resonate 40 years later, says Kweli, who was stopped in an airport after 9/11, by “the FBI, the CIA, and the TSA,” because the government knew that he had been listening to Carmichael’s speeches. What these agencies and other authorities who remain fearful of black power miss, says Kweli, is that Carmichael, a “fiery speaker, “was “just a regular dude.” Exhibit A, for Kweli, is the interview with Mabel Carmichael, where the son respects his mother’s strength and dignity."
http://www.popmatters.com/review/148310-the-black-power-mixt...
(Highly recommend the movie, as a wonderful window into the recent past)