At first, when I came upon this, I thought it meant it elevated buttons for display. But this did not make a lot of sense at first.
Thrift store staff and customers tend to be a sociable and helpful bunch. After a bit of discussion the consensus was that the item is used for hand sewing buttons onto garments as a guide for fine fabrics like shirts to heavier like pants to heaviest like coats so the shanks of thread will be of appropriate depth and equal in length.
Wow. So now I know. I actually do not recall the last time I sewed a button onto fabric, thick or thin, but the nifty little tool is now in my sewing box ….. just in case. And another use may come to mind. I do have some very nice buttons and could elevate them for display.