Matt medium is used for many things. I used it here because I was thinking about using oil paint somewhere in the process, and mat medium sealed the surface of the illustration board so the oil paint would not come into contact with it directly. Plus, mat medium is a joy to paint over top of because, unlike gesso, it drys clear and does not create unwanted texture.
You can also work right over top of matt medium with pretty much anything you like. In the image below I started to render the face in colored pencil before moving on to acrylic paint.
