Sarah Stern:

It will come in handy if you ever misplace your device, or if someone were to steal it. You can use the app to locate your device, send a warning message to the culprit and sound an alarm or beacon. You also have the ability to remote wipe all the information from your device and create a passcode from any other device with the app installed.

Find My iPhone is now 100% free, no strings attached. There is no point crying over spilled milk; it only works once you’ve gone into Settings.app and set it up.