Back in 'ye olden days, people used to print out programs... nay, they even used to _hand-write!_ programs before they began typing, because keyboard-time was valuable (nevermind compilation/computation/debugging time).
Serial ports were slow, grep wasn't really a thing, so having a printout (or "listing") of your program was a more efficient way (or only way!)to debug your program after the fact. https://www.youtube.com/watch?v=tJGrie7k97c
Back in the 90's, I had some programming classes in high school where there were 30 chairs, but 15 computers (around the edge)... bring your own 360kb floppy disk! So you had a real incentive (and a strict teacher) who insisted that you wrote out your program ahead of time, show it to her for a first-pass/feedback, and _then_ you'd get to go type it on the computer and see if it worked. Submissions were via printouts (of the program, aka "listing", along with the output) which she then took home and graded.
The whole document IMHO is worth a read, but is definitely a product of it's time (70's/80's/90's). https://en.wikipedia.org/wiki/The_Tao_of_Programming
Stick tongue firmly in cheek, empty your cup, and enjoy the ride!
Edit: ...and the relationship to the cantankerous original comment who "couldn't figure why they'd want to run OSX?", this is the zen-koan sarcastic response of: "use it as a platform for development" (ie: stack your papers on top if it)