Citation, Documentation of Sources

Q. I am wondering why one needs to provide the URL for a journal or newspaper if one consults it online, but not the name of the library, say, if one consults it in print form? Typically everything about the articles is the same, and so the place where one found them should be irrelevant. Indeed, if I understand the logic, if one downloaded the PDF of a book, one would need to provide the URL, but if one made a PDF of a book and then read that, one wouldn’t have to. What am I missing?

A. The name of the library where you found a book won’t be much help to anyone who doesn’t already patronize that library. Sharing the URL for a source you consulted online has the potential to give readers instant access to that source.

[This answer relies on the 18th edition of CMOS (2024) unless otherwise noted.]