My buddy recently spent a couple weeks in Europe, traveling with friends. He went there for Oktoberfest then visited Italy, the Netherlands, and France. While in Paris, someone stole his buddy's iPhone. They tracked it down to a street corner where they found a group of hooligans (there's a word I rarely use); when confronted, the kids ran off.
My buddy and I both used to work at Apple, so we discussed what someone could do with a stolen iPhone. We concluded that a stolen iPhone is useless without the login and password, even if you replace the SIM card. Around 2010, iPhones and iPads were becoming hot items, making them a key item to steal. A common problem on the NYC subway was that a thief would snatch it out of a victim's hand when exiting a subway car as the doors were closing.
In response to the rash of thefts, Apple began making it impossible for an iPhone (or iPad) to be used without the proper credentials. These devices can't be reset to factory settings; so, if you forget your PIN or Apple ID login and password then your device is bricked.
UPDATE 8 Nov 2017: I asked a question at this morning's Tech Coffee, "What good is a stolen iPhone?" and an entrepreneur answered with "Spare parts."
Can iPhones be stripped like a stolen car? Perhaps the screen?