Wendy, my guess is that say pre-1950/40 the Rural Free Delivery (RFD) postmen or letter carriers all had a set of scales like that to insure that the letters they picked up had the correct Postage Stamps on them and/or that the person mailing them had left the correct amount of money in their Box with the letter that they wanted to mail.
Most rural families never actually purchased a Postage Stamp, they would just give the letter carrier the money and the Stamp would be placed on the letter when it got to the Post Office. Given said, it saved a lot of confusion, time and returning of said letter to the sender if the correct number of pennies were paid in the 1st place.
Anyway, I’m also pretty sure that most of those reproduction Postal Scales being sold today are actually used for weighing Wacky Tobacco and other illegal drugs, ….. but being they are ACTUALLY new versions of old Postal Scales the Cops or ATF can’t really bust one for having “drug paraphernalia”.