They reduce handling costs tremendously, and you no longer have to teach people how to count to use them. That task can be left to the public schools and institutions!
Are you talking back in 1975 for the A-series, or the UK permanent NVIs in the 80s?
BTW, make as much sense as the ONE PRICE PRIORITY MAIL or EXPRESS MAIL cartons.
It's funny that we're gone back to NVIs after so many of us used them in our childhood playing "post office". But with the economy the way it is, it makes sense.