The most important practical advantage -- one reason I like the A4 / A5 family -- is simply that proportions are preserved when you print two pages on one sheet

The most important practical advantage -- one reason I like the A4 / A5 family -- is simply that proportions are preserved when you print two pages on one sheet and fold over. Adjacent to that is the advantage that there are a family of sheets to work with in practical sizes -- A5, A6 and A7 are all quite useful -- and they bear a certain logical relationship to one another that is easy to work out, in terms of how many of one size fit on another size.