@Andy Set designers are typically reflecting back the zeitgeist of their day. They're attempting to answer the questions: "What would these characters have in their environment?" And by extension, "what would a prototypical person have in their environment?" In other words, what would make this look "office-y"? What would make these guys look legitimate for their ages and professions?

As an example, one of my own zettelkasten card index files from the early 1900s was acquired for use as set decoration on the 1982 NBC television series Cassie & Co. starring Angie Dickinson. As a private investigator, Dickinson's character ostensibly used the filing cabinet as a database as many offices did throughout the 20th century prior to computers taking over this office function. At the time, it would have said, "I'm an old-school detective and don't have the income or savvy to be using the cutting edge computers you might see in other television shows." Because it was older, original oak and not a more (then) current steel, it would also have said "I've got a bit more style, have been around the block, and I've been doing this long enough that I'm not the new kid." Currently I'm giving it new life with similar functionality despite the fact that many simply wouldn't understand the use-case anymore, as evinced by many selling them as cabinets for tools or various sundries without even knowing or mentioning them as originally for index card use.

Another fun example of related technology in set decoration can be seen in the Hepburn & Tracy picture Desk Set (20th Century Fox, 1957) which pits old school research and knowledge management methods like typewriters, filing cabinets, card indexes and the human brain against new computer methods. Given its time period, it has a lot more filing cabinets and card index filing cabinets. (As a movie, I think it still has something useful to say with respect to the ascension of artificial intelligence today.)

Here are just two screen captures of the shift in office set up and technology which are interesting for means of comparison of "then" and "now":