The oft-cited reason is that you can fold A4 in half, etc. to form other standardized paper sizes.
Which sounds nice, but in practice just doesn't come up all that often.
Photocopiers and printers outside America include buttons for this.
It's a standard feature of photocopiers in the US too.
Papermakers definitely benefit from this standard, which indirectly benefits you.
And from personal experience, being able to turn any stack of A4 papers into an A5 'zine is great.
