I have one. I cut the buttons of off clothes that become rags. When clothes come with extra buttons they usually go into the button jar too.
Oddly, I've never used any buttons out of the jar yet.
I strung all of my old extra buttons that came with clothes for my DD when she was little. I also have a button box, but it's mostly stuff I bought. They are mostly on cards. I always check there first. I don't cut them off of clothes. I usually donate old clothes before they become rags, so I don't need to cut the buttons off.
I am still oddly attached to my grandma's button tin. I love to go through them, and even weirder, love the smell of it! Brings back many happy memories. I think this year I'll start my own button jar! I actually think a clear glass jar (Ball canning jar type) with buttons would be a beautiful knick-knack on an end-table!