You got bonus grade and doing them all nets you a event and costume. Ultimately they're just an annoying pain and the remaster lets you redo the Secret Mission fights anyway so there's no point in stressing out about them until you're in the post-game or whatever.
Going through this game again, I don't think I can agree that the soundtrack is "boppin". It's kind of...standard? It's not bad, but I'm not rocking out to any of the tracks. Out of the Tales games I have played, I think the only soundtracks that have stood out in any way were Destiny 2, Rebirth, Symphonia and Legendia. I cannot even envision any of the tracks from Graces to Berseria in my head.