Having locks rekeyed takes existing locks and modifies them to function with different keys. Since no new hardware is purchased or installed, having your locks rekeyed is often more cost-effective than having them replaced with new ones. Your current locks are simply adjusted.
There are many reasons to have your locks rekeyed. Some are for safety and others are for convenience.
There are many reasons you may want to prevent old keys from working with your locks:
If your locks are compatible with each other, you can have multiple locks rekeyed to function with the same key. Many people, for example, have this done in their homes so they can have their front door, back door, and garage door all work with the same key.
Some locks are capable of accepting more than one key, in which case they can be rekeyed to accept both separate keys and a master key. This is called a Master Key System.
In general, you should rekey your locks if you are content with your current locks but need to change which keys will work with them.