[jacobolus]

ISO paper sizes assume that content is scale-invariant.

US paper sizes assume that you have specific design sizes to your printing technology (picas) and then give you a grid of an even number of those units.

Graphic designers / typographers I know tend to prefer the US paper sizes.

Regular people who want to make photocopies of enlarged/reduced pages tend to prefer ISO sizes.

[einhverfr]

Totally misses what ISO paper sizes are designed for, namely book binding and signature printing. That's why the scale-invariant approach is so important. Once you know how many leaves in a signature and what page size you want, it is easy to figure out which paper size you want to print on.