Fading is not one thing. It is the combined work of ultraviolet radiation, solar heat and visible light, and ordinary window glass lets a great deal of all three into your home. UV is the biggest single contributor, which is why it is the first thing a quality covering is built to stop. Heat and intense visible light do the rest, slowly cooking and bleaching whatever sits in the sun path day after day.
The materials that cost the most tend to suffer first. Hardwood floors shift colour unevenly, leaving pale rectangles under rugs and furniture. Artwork, photographs and textiles lose their depth. Leather dries and lightens. By the time fading is obvious, the damage is permanent. The only fix is replacement or refinishing.