The Shelter Key is one of three items required to unlock the secret ending of Deltarune Chapter 3, but it's easily missed.

Where Do I Use The Secret Room Key? After completing the credits and returning to the game, you should return to the Past timeline and go upstairs in Eventide Krat Hotel to the Gold Coin Fruit Tree.

Three types of Keys are items you need to get in Dragon Quest 3 HD-2D Remake to progress the story and unlock new, important areas around the world.