There are a few mobs from the underground that have unique drops (bats with bat wings and worms with worm guts), but from my understanding worm guts/bat wings have no real purpose. It would be nice to have something along the lines of a relic or jewelry slot under the player’s armor tab.
Bat wings could be used with some other materials, or on their own, to make a relic that increases player movement speed.
Worm guts could be used to make a relic that increases mining speed, and possibly even a % chance to receive more materials than normal when mining.
More relics that increase other stats could be found in chests, making for exploring the world and visiting the underground more worthwhile.