I have a button jar too! It contains some buttons leftover from sewing projects, some that my MIL gave me and quite a few that have been salvaged from old clothes! It's a habit I picked up from my mother and grandmother.
So few of my friends and colleagues sew that I wouldn't be surprised if they knew nothing about button jars.
Buttons have been used in various sewing projects; but when the kids were younger they were used for quite a few other non-sewing purposes. The buttons became makeshift pieces for board games, pretend coins etc. They were also used for teaching math concepts - sorting, addition, subtraction, times tables etc.
Nowadays, a lot of the buttons are used for craft projects.