The reason people should re-key their locks include; a tenant moves out, a key is lost or stolen, your purse is stolen, new construction you don't know who on the construction team still has a key, you just purchased a new home or moved into a new home as you don't know who still has a copy of the keys - Realtor and/or previous owners. One of the most popular reasons is to have all your locks using only one key. This makes it convenient for you to not carry a lot of keys around with you. This can only work, however, if all your locks are of a compatible brand in your house. (Kwikset, Schlage, Weiser, etc.) Sometimes when hardware is installed into your house you can purchase them to be "keyed alike". This means that all the locks operate on the same key. However, if you decide to add a lock later on, you can either re-key that lock to be on the same key as the rest of your doors or use two different keys. (Remember this only works if the hardware is compatible.)

One of the other reasons people choose to re-key is to prevent theft from someone who had a key but no longer works/lives there. This is less expensive than replacing all the locks. It invalidates the previous keys from working. This also serves as a preventative measure to theft. If a person has a key and steals, then it is more difficult to 'prove' that they did it. After all, they had the key, that meant that they could come and go. If you change the locks, then it becomes a felony charge of breaking and entering - which holds a much stiffer penalty.