Glazing is any protective layer of glass, acrylic or laminate that covers the artwork. The glass helps protect the artwork in many ways. It is available with a UV (ultraviolet) light screening, which is a coating on the glass or acrylic that cuts out up to 98% of the UV light. Glass also comes with different non-reflective coatings or treatments which helps cut down on the glare from natural or artificial light
Light can cause extreme fading of colors and also interacts with the acids and other chemicals that might be present in the mats and in the frame. It does not have to be direct sunlight to cause extreme damage.
While UV protecting glass is not a cure-all, it does significantly extend the life of most paper art. But if you hang the artwork where it will get direct sunlight, moderate fading will occur over time. There are also other light rays that cause fading and damage that are not cut out by the UV protective coatings.