Composition refers to the organization of pictorial elements in a frame. Every image should have a single story to tell. The purpose of composition is to direct your viewer's eye to the central point or "story" in your scene. Regardless of the particular subject, composition of the frame is important. To present a picture to your audience that is unified, shows the relationship between the elements in the frame, and is aesthetically pleasing. Everything in a frame is important, not just the subject!
There are two ways of viewing a film: passively and actively. The passive viewer sits back and enjoys the show, rarely asking why particular choices of composition or camera placement have been made. The active viewer not only sees and hears the film, but also is involved in exploring it on a deeper level - one that is not immediately apparent to the casual viewer.