Less the "imperial" part and more the "almost whole"---it would work just as well if the pages were say 20cm by 30cm---but it is very convenient when doing layout (both electronic and physical) that both the content and the margin are "nice round numbers", and the natural divisions are also nice round numbers that you can easily measure with a standard ruler. 1/2" margins all round? Then the area is 7½ by 10. Printing a 2-up booklet? Each page is 5½ by 8½, exactly. If I had to design the size from scratch to optimise this property I'd probably go with 8"×12" (or 20cm×30cm!) but I assume there's some older printshop standard we're inheriting from here.
Not saying A4's not good. Just acknowledging that "nice round numbers" are convenient sometimes. Standards that use US-traditional measurements seem to do better at that for some reason.
Yes, I get how the A-series paper sizes work, and that they prioritise having all sizes be the same aspect ratio. That has literally nothing to do with what I'm talking about in my post. It turns out that paper "works" for things other than resizing other sheets of paper.
Not saying A4's not good. Just acknowledging that "nice round numbers" are convenient sometimes. Standards that use US-traditional measurements seem to do better at that for some reason.