When you lose track of your keys, you should consider rekeying your locks to avoid a possibility of break-ins in case the key is in the wrong hands. Rekeying changes your lock configuration such that the lost key won’t open it. Also, you should consider rekeying door locks after moving into a new house. Even when the previous occupants return the keys, rekeying prevents access into the house in case they might have copies of the key. More can be found here.