When I was a wee-little-kitling, aka, a high-schooler, I took a ton of music and music theory classes. I loved my teacher. She was amazing! But that's besides the point.
Music, as with writing, helps to express the emotions of the world. One song, played on the radio, has the advantage of reaching millions of people at one time, where as an author works at an individual level book by book (except for War of the Worlds, which kinda proves my point about mass audiences...).
If we look at movies as well, film and television, a piece that has a killer soundtrack often helps to influence the impact of the show.
Take, for instance, Game of Thrones, and yes, okay, fine, another one with a mass audience through a visual media stream, I GET IT! lol. Anyways, the moment you hear the first two notes of the opening credits, it sets the entire theme of the movie. Pirates of the Caribbean. Harry Potter. Lord of the Rings. Anything by John Williams. The films are made by the music that imbues them with character, above and beyond what the actors are doing onstage.
But the point is, composers, arrangers, are taking some story and setting it to music, amplifying the emotions present within the written word to affect a wider