I prefer when it plays a song as the battle theme that is a version of what you've been hearing the whole time, where the lyrics start making sense and add a bunch to the fight.
Ex: FFXIV Shadowbringers, the theme "Shadowbringers" that starts out "One brings Shadow, one brings the Light..." and is just generally a banger that goes along with a great and meaningful fight.
Also the quest you're completing is called "Shadowbringers."
So kind of the same thing, I guess.