The "necessity" of a theme song in a movie?

While I won't say it's a necessity, but a theme song can be a powerful storytelling tool and when used properly, it can really benefit a movie. Think Star Wars. Not everyone held a high opinion of the film while making it, but when John Williams composed the theme that opened the film, it gave the film a victorious, symphony-like musical sound that gave the film some weight. I don't think Star Wars would be as popular if not for the musical themes.

In the end, they are essential for character symbolism and introducing the mood of the film.