Card Catalogues.

Books The Folger Shakespeare Library has still retained its card catalogue. In an age when "everything is online", more often than not it isn't because in some cases the process of transforming some texts isn't cost effective or there's a balance to be struck in relation to whether it's even worth the attempt due to how much it would actually be used.

Here they mount a defence for why it's still important in 2016:

"... card catalogs provide an opportunity for serendipitous discovery that is difficult to duplicate in an online catalog. The physical act of searching through the cards requires a somewhat different thought process than searching an electronic catalog. Flipping through the cards on your way to what you think you’re looking for is a great way to find items you didn’t know you were looking for. Because the cards can be physically arranged in different ways, it is possible for us to provide many different access points into our collection. For example, we have both chronological files and publication location files."

Yes it's true. Keyword searches in OPACs are fine as is visiting a shelf number and browsing the materials clustered around the book you're looking for, but there's serendipity to card catalogues, to randomly visiting the drawers and seeing what you'll find.