The best advantage, which you might not realize until you need to use US sizes, is the easy scaling up and down. You can put an A3 sheet on a photocopier and p

The best advantage, which you might not realize until you need to use US sizes, is the easy scaling up and down. You can put an A3 sheet on a photocopier and press the "A3→A4" button and have the original perfectly scaled onto a single A4 sheet, or vice-versa. There's usually a button that does A4→2 A5, so you get two half-size copies on the A4 paper, which you can cut into A5 sheets. (In my school, university and workplace experience, it's very common for A3 paper to be loaded in photocopiers and some printers.) Equally, you can design something in A4 size on the computer, then print it on A3 paper without any changes. Then send it to the print shop for an A1 print, still with no changes. An envelope that fits A5 paper will fit once-folded A4 paper perfectly. (Magazines people expect to be photocopied, like scientific journals, are often A4 sized.)