The Secret Key unlocks the Cinnabar Gym. As long as this item is in the Bag, the player is freely allowed to enter this place, where they can challenge Blaine and earn the Volcano Badge.
If the player does not have the Secret Key:
In Pokémon Red, Blue, and Yellow, if the player steps on the tile at the front of the Cinnabar Gym door, the message "The door is locked..." appears and the player automatically walks a step away from the locked door.
In Pokémon FireRed and LeafGreen, the player no longer walks away from the door automatically due to the lack of this item, but interacting with the locked door still returns the same message as in the previous games.
In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, interacting with the locked door returns a longer message explaining that the item to unlock the door is found "somewhere in a building overgrown with greenery".
