Art has always been intersectional. Where one type of art begins and flows into another comes from the variety of ways that artists and the general public produce and consume it. While it can seem arbitrary, art both sonic and physical is both objective in ways and subjective in others.
One form of art usually needs another to elevate it. Films have long used well fleshed out musical scores to impress the heaviness or lightness of a scene or sequence. Orchestras often accompany ballet, and contemporary dance performances while visuals may play in the background.