Good sound design adds to the emotion of the scene. From the creaking of a door in a horror movie to the epic score in a fantasy movie, sound matters. The score can elevate the mood of the scene. There are also many tricks in composing music that is more sound design than music. One of which is using scales that don't resolve to add tension to a scene and reflect the environment the scene takes place in.
The Vox discusses this technique in more detail in the video below: