Mat decoration originated as the artists desire to set their work apart, to direct the eye to the image. Although the earliest borders were on the art itself, they soon appeared on the papers used to mount and repair. Not until the development of glass was art on paper displayed in any great quantity.
The first cardboard mats date from the industrial revolution in the early 19th century. The cardboard was hand-covered with artists' paper by the framer.
Framing with ready-made mat board as we know arrived in the late 1800s.
The mat's purpose has always been the same: to create a favorable surrounding for art. This is as true today as it was in the beginning.