Knowledge is power; the Wright brothers flew because they understood the laws of aerodynamic. Likewise, understanding the "rules" of composition sets you free. It doesn't obligate you to use all of them all of the time. Knowing them well, you'll know when you can safely ignore them. You'll use the rules not merely to prevent or solve problems, but also to have fun thumbing your nose at them. Degas could challenge the rules of composition because he know them well - you could say they spent a lot of time together.