Reasons to Mat:
First: Originally mats were developed to put a separation between the art and glazing (glass/acrylic). Glazing should never be laid directly onto the art because moisture can build up between the glazing and art causing the art to stick to the glazing. This can cause permanent un-repairable damage to the art. In some case a spacer can be used instead of matting.
Second: Mat can be a decorative element to emphasize or de-emphasize certain colors, or to coordinate with the decor. Generally the top mat should have less color intensity than the colors in the art. Some people mistakenly believe that a wide mat overpowers the art. That is not true, if the mat overpowers the art, it’s because of distracting color, not width.
Third: Proper mat makes the entire frame assembly larger and gives the art more visual importance. A wider mat creates more visual importance. Conversely, a narrow mat denotes less importance. If a framed piece of art is to be a focal point in the room a wider mat should be used. If a framed piece of art is to be less visually dominant and blend with the surroundings, then a narrower mat or no mat at all, may be more appropriate. The choice is yours, and should be based on your intentions of the art placement in terms of visual importance.