> I've never seen anyone (in a real-life use case) resize their browser window to be narrow on their desktop,

I do this very very often.

I will be watching a YouTube video or a BBC iPlayer video in one window. Then I'll open another browser window and resize it to be tall and thin.

It is infuriating just how poorly most website cope with this tall thin window. And really, most text is easier to read in a narrow column.