I read Cecil’s explanation of why U.S. letter paper is 8.5x11 inches, after which he blew off metric paper sizes as “obsessive” and created by Germans.
There’s one very handy thing about “A”-sized paper. (By the way, there’s “B”-sized papers as well.
All “A”-sized paper has the same proportions. Thus, you can print a hardcover on, say, A3, and put out the paperback on A4 by simply blowing down the text. The layout remains constant.
In the U.S. over the last decade or so, printers are finally starting to use the system the rest of the world relies on. This causes some problems, e.g. books too big for standard shelving, but the benefits far outweigh the hassles.
“Obsessive”? No, smart.