This computer is about eight years old and has been through I-don't-know-how-many OS updates. This, and the fact that not everyone has these files, makes me suspect that there is supposed to be some mechanism cleaning these files up, but that the details have changed over time, and some OS update got it wrong.
You can view (some) of it with: 'lpstat -W completed'; add '-l' for more detail. Add '-u USER' to view completed print jobs of another user (there doesn't seem to be any way to see _all_ user info, cups' command line tools always seem to have aggravating shortcomings like this...)
FYI the default value is 24 hours so it might be interesting for you to print something now, see if the job file (starting with d, I guess this is sort for 'document') is preserved after the print job completes, and then check a couple of days later to see if it's been removed by cups.
[subsequent edit] Maybe you ran into https://github.com/apple/cups/issues/5747, "You really need to use a current release of CUPS. I suspect this is due to a known bug that was fixed a long time ago" - maybe that bug shipped with an older version of macOS and that explains what people are seeing.
I also noticed that there has been issues, like https://github.com/apple/cups/issues/5538, related to spool file clean-up. (The fix to that one leaves some decidedly fishy-looking parentheses, but they were fixed in a later commit.)