California was the first US state to require new smartphones to have a built in Kill Switch. If the phone is stolen the owner can log into their user account online and lock the phone with a message appearing on it reporting it as lost/stolen. If the phone isn't recovered the phone can be "bricked". Bricking wipes the phone and makes it unusuable.
The intention of the Kill Switch is deter smartphone theft but an article from the Wall Street Journal shows it has had mixed results. In Austin, Oakland and San Francisco thefts have dropped between 11% & 38%. While in Seatlle thefts have increased 32%. There are people being tricked into buying bricked smartphone and onlince service claim they can unlock them.
The Kill Switch might not be the ultimate deterrent to theft but the Kill Switch has an important benefit. Bricking the phone wipes it of all its personal information stopping the thief from accessing any sensitive data that could be used to hack personal accounts.