Photographs don`t generally follow the rules of colour theory for example (and that includes composition). That's true, but through the eyes of a skilled and artistic photographer, you have a photograph that makes a statement, or that conveys an emotion that is intentional. That is the difference between an everyday photograph and a photographic work of art. You're right .. you don't see color theory and composition in nature. But here we are talking about art, which can convey anything from fantasy to photorealistic images. With or without the rules, smart use of color and composition can project a feeling or a mood, or make the eye travel through an image in exactly the way the artist intended. Some come by this skill naturally, while others learn it. Where color theory study MAY be useful is in clothing and makeup texturing, where it also occurs in the "real world" as well. Texture artists may find color theory very interesting and beneficial.