I’d guess that nomenclature originates in the world where every small US Main Street had a stationary store carrying all manner of paper sizes and stocks for diverse purposes—none of which involved use in anything more sophisticated than a typewriter.

One particular “standard” that sticks out in my memory was “math paper”, which I recall as being unbleached, about 5” x 8”, and used pervasively in primary education (at least in New England) into the 1990’s.

Oh... I don't really see why "letter" is a more confusing way to describe a paper size than "A4"...

My general point is just that I'm surprised so many people seem to notice and care about paper size in general. I've just never thought about this at all.

Well, "A4" doesn't imply anything about the intended use. The format of the name also implies that there is A3, A5 etc, both of which aren't all that uncommon either.

But, yes, for most people it doesn't really matter - you go to the store, you buy paper, you shove it into your printer, and it mostly just works. However, it's also not all that hard to run into situations where things break. E.g. most PDFs originating from US are rendered for Letter size paper, which means that printing them outside of US generally requires setting "fit size" rather than "original" to ensure that nothing gets clipped. Vice versa also happens, but because US is so culturally dominant, Americans rarely run into that particular issue.