We’ve had a restricted key section for many years so that we can build locks and make master suites etc. When using a restricted key, security is ensured as no-one but the person who requested the locks / keys can have extra keys cut. Anyone trying to get a duplicate copy of the key from shops in town etc will find that nobody can order that keyblank to be able to duplicate the key.
If you hand a key to someone, and a while later get it back from them, you are totally confident that there is no other key held by them as they can’t have got it copied.
Additionally, the keys are thicker making them stronger. Far less easy to bend (I really don’t know what some people do to their keys – I swear that some must use them to pry paint tin lids off or something). Some people are known to not look after a key…
It should ONLY be used for locking and unlocking the lock it relates to, but that seems to be ‘news’ to some people.