Chest Lock

Locking a chest yourself does nothing to the items inside. It just puts a key requirement in front of them. The main use is achievement hunting, because a chest you locked counts the same as a naturally generated one when it is opened again, so the dungeon and biome chest achievements can be earned in a world where you already looted everything years ago.

Anyone carrying one can lock a chest you are actively using, and the contents then sit behind a key nobody has spare. Keep anything you care about in a container that cannot be locked, or in a house other players are not wandering through. Biome chests are a special case: they can only be locked once Plantera is down, and only if they somehow got opened before that.