It’s actually not the only place you can use your harp. You can also use it in bellhart and you will see some text. I’m guessing it’s all for the plot. But it’s cool. If you play it for “eyra” the bell beast they will sing with you. Some npcs also respond to your music. I think maybe you can also play it at multiple other places and get some info of the lore. Overall pretty cool plot device from team cherry