Back when jackets still closed at the top, the boutonniere was a functioning buttonhole. Its purpose was to allow closing your jacket all the way up in cold weather. Believed to have originated in Italy and then made popular by Italian tailors who relocated to France.
Nowadays only few tailors are able to make such impeccable buttonholes since they are challenge to properly execute on. Milanese buttonholes require extra labor and a higher cost making them rarely seen these days. The offering of a Milanese buttonhole is something only a quality tailor can provide. Where machine made buttonholes have the lapel buttonhole stitched by machine and cut afterwards, the Milanese must be cut first and then carefully hand-stitched after. It involves a piece of thread called a “gimp” which is hand-stitched to surround the buttonhole.