I believe that kill switches. In my area at around year 2000, when 'dumbphones' were common but still rather pricey, theft or mugging of phones was a common crime. It dropped significantly overnight when IMEI blacklists were implemented and reported stolen phones wouldn't work on local networks. There were still some thefts of luxury phones, where it made sense to export them abroad, but it was a huge reduction.

Even if kill switches can be circumvented and phones can be sold for parts - it significantly reduces the incentive for someone to steal phones, as selling them is more effort and less money; such changes do affect behavior of criminals.