For digital printing this standard is very important and handy especially at home/work. 1. Not really - if you go to professional digital printer they have oth

For digital printing this standard is very important and handy especially at home/work. 1. Not really - if you go to professional digital printer they have other formats +, RA and SRA (A3+, RA3, SRA3) that are slightly bigger to accommodate for bleed. 2. It matters if you want to make saddle stitched binding yourself - you fold A3 and get A4 booklets. 3. If you make bigger runs of anything at professional (offset) printer the standard sizes don't matter that much because you print on big sheets (like B0) that get cut (often multiple different jobs on same sheets). So it's about waste (not printed space that gets trimmed). But the paper waste really matters only in very big runs 5k+. Whats more expensive are offset printer plates that need to be made. So yes the magazines/books can be any size but the space on rest of the plate/paper will be filled as much as possible.