The earliest identified button is a pierced and curved piece of shell, thought to be 5000 years old and was used as an ornament - buttons were not used as functional closures until approximately the 13th century.
Here's the entire set, displayed an a copy of a Victorian button card - even then it was all about the presentation.
The larger button would be purely ornamental, perhaps to accent a belt or to trim a hat. The mid size button could be used as either functional or ornamental on a substantial fabric such a s velvet of brocade. The smallest button would be stunning used down the back of a bodice - just picture them all in a row.
The choice of buttons can make or break an outfit and it was not unusual for a period outfit to have buttons created specifically for that particular garment - it's all about the details!