My friend's mother did custom upholstery for the mega yacht industry. She had some metal button covers left over so she sold them on ebay. They went so fast she decided to go into the button cover business. They were selling $700 - $1000 a day worth of cover buttons out of their house. I did the orders for a couple weeks while they were on vacation. It took me 2 hours a day to get all the orders out. The manufacturer died and the family was going to close the business since nobody wanted to buy it. My friend's mom and the whole family decided they would either have to give up their business or buy all the equipment and do it themselves which is what they did.
Basically once upon a time "luxury" buttons were either of precious materials (like solid, real gold) or sporting a precious stone or an enameled surface/design and those owning dresses with these buttons could afford to have them removed (and later re-sawn) when washing the dress was needed, as there was the possibility that they became damaged.
Using more common metals (like brass or similar) poses another possible risks during washing, the fabric could result stained by some reaction of the soap/chemicals involved with the metal.
Metal button covers are thin covers that you can put over a normal (usually plastic) button to give them a particular "look", and they can simply be removed when washing is needed.
