For when you no longer want to find a specific iPhone — or iPad or Mac — any more.
Find My iPhone lets you keep track of all your iPhones, iPads, and Macs, all in one place. If you're upgrading to a new device, trading in your existing one, or even selling it, however you'll want to make sure you completely remove it from Find my iPhone before letting it out of your possession. This ensures the new owner can't access your data and that they'll be able to link the device in question to their own iCloud account.
You can disable it directly on your device, or remotely using the web. Here's how!