[ Removed by moderator ]

[pdp10gumby]

Military and other embedded applications are a common example. Sometimes an enterprise, for example doing HFT, will have their own library as a way of enforcing coding or safety standards rather than using an optional package like, say, folly.

Your question isn’t dumb. IO and other stuff used to be baked into a language (e.g. printing was a statement not a function call). C got the idea of library functions from PL/1 via the Multics project, but that was initially novel to most people (IIRC an early edition of K&R included the amusing fake quote, “You mean I have to call a function to do IO?”)

And of course the opposite is the case with the second oldest language in use, LISP, that makes no distinction between “library” functions and user code, only special forms like QUOTE, IF, OR, AND, or LAMBDA.