"Engaged frames" are more common on larger works. All the moldings for an engaged frame were made separately and attached to the panel before the artist himself ever began to paint. This work's pointed arches, triangular gables, quatrefoil shapes, and other decorative motifs were inspired by contemporary Gothic architecture. They were intended to enhance the sacred aura of the altarpiece and the painted holy figures when illuminated by candlelight in the often dim interiors of churches.
Much of what we see in Gaddi's work is original. Some pieces, including parts of the base and the spiral colonnettes, were made and added in the 20th century to replace missing parts that originally covered joints between the painted panels.
Learn more about early frames: http://1.usa.gov/1b5I5lv
Agnolo Gaddi, "Madonna and Child Enthroned with Twelve Angels, and with the Blessing Christ [middle panel]," shortly before 1387http://1.usa.gov/18EvVOo