Film came before television, so it follows that the development of TV took several cues from film. It makes sense to many because so much of what appears in theaters ends up on your living room television months later.
In the 1950s, film makers changed the aspect ratio they use, opting for a film that is much wider than it is tall. It makes sense, because the human eye has a greater field of view left to right as compared to up and down. You can see the difference wider pictures make when you go the movie theater. This is also the reason why you see the message "This film has been modified from its original version. It has been formatted to fit this screen " before many of the movies you watch on your television. The edges of a movie's picture are cut off when they are shown on TV unless the movie is shown in "letterbox" format. Letterboxing a film on home video and DVD presents the picture with black bars above and below the image. Letterboxing reduces the size of the overall picture shown, but you see the entire image.
In keeping with the movie theater tradition, HDTV uses a 16:9 aspect ratio. With your new digital HDTV set, movies are shown in letterbox format, but without the annoying black bars above and below the picture.