Photography is an art form, and just like every other form of visual art such as painting, drawing, or collage, it follows the same rules of composition that the other arts do. Although they are called “rules” they are really more guidelines because, obviously, you aren't required to follow them.
Different subjects often require different approaches and often many of the rules of composition overlap as well.
As I said, you aren't bound to the rules of composition, but when you're just growing accustomed to any visual art, learning these rules and following them will help you learn to create images that are above and beyond general snapshots so you can make professional-looking photographs. After a while you, when you begin to fully grasp the concepts, you will find yourself innately applying them to your compositions.
Simplicity is one of the great keys to making a strong image. An image that has a succinctly defined subject commands attention much easier than a composition in which the viewer must scan the image to determine which part is most important. Images that contain a variety of competing elements can be distracting and cause the viewer to lose interest.
One technique professional photographers use to create simplicity in an image, especially in a busy environment, is to use a wide aperture to create a shallow depth of field, isolating the subject from the background. A shallow depth of field creates blurry, out-of-focus ...