When an iPhone is stolen you can set it in lost mode remotely, which renders it for anyone who doesn't have the username/password. It can't even be wiped without the proper credentials.
https://www.intego.com/mac-security-blog/how-to-enable-the-kill-switch-on-your-iphone-or-ipad/

Thefts are down as a result: http://www.cbsnews.com/news/iphone-thefts-down-thanks-to-apple-kill-switch/