The thing I love about the ISO paper sizes, and in particular the A-size papers is that A0 is the biggest, if fold that page over you have two A1 pages, if you fold one of them over (after cutting it) you have two A2 pages etc.

So if you have an A4 printer it is really easy to print things on A4 and cut it down to work with something A5 or A6.

For other sizes you always either end up with waste or weirdness depending on if the printer lets you do weird things.