Skeleton Key Lore
Back in the olden days, when housing was more available, all homes had a key for every single door. If you had 25 doors in your house, there was a ring of 25 keys or a ring of keys for a floor of doors, tucked away in a cupboard near the front door or the kitchen. Eventually, a 'Master' or the Skeleton Key was developed for the owner of the house so that all the doors can be opened with one single key.
From a magical perspective, if one key can open every door on the physical realm, it can open every door on the energetic level. If you were enslaved by someone who held the key to your freedom, obtaining it would help you gain your power back. Claiming a key for yourself can be worked to open all opportunities from prosperity, success, even locking away the past.