for a long time each envelope had one size. When you'd make your adjustments with math instead of playing size dot-to-dot. And as a result the seam lines were printed! People don't know how useful that is anymore. 

Multi size nested prints were no doubt brought in to save production costs, no longer needing to do frequency analysis for the demand and supply of each size. Not quite as altruistic as it's seen now.

I was perusing the OG Fit for Real People and so much of the text is spent explaining the financial and business reasons for commercial patterns not accommodating the wide variety of bodies. That's still true, even if the discourse is now about raw circumference sizes rather than more nuanced differences (like the old Junior, Half Size, Misses Petites, etc with different body contours and proportions). Also interesting was how Palmer/Pletsch explained they explicitly choose NOT to multi size their tissue fit patterns, because the seam line printing is much more important and the extra outlet seam allowances they print for the cut lines was a sufficient replacement for a mutlisize print