This is simply an additional piece of mat board to which the art is fixed, usually two ply. It is common to mount the art directly to the backing board, but you should keep in mind that whatever your artwork is mounted to essentially becomes part of the artwork. If your backing board is damaged, for instance, you’ll probably want to detach the artwork and re-mount it. This can be difficult, particularly if you have spray- or dry vacuum-mounted. The backing mat is especially wise if your art will be travelling- frequently handled art is frequently damaged art.
The highest level of archival conservation does call for a backing mat- but Westfall Framing does not recommend a backing mat for every application. Considering the work’s value, the expected life span of the piece, the potential for frequent handling, and even whether it may be reframed can help you decide on the backing mat.