According to the head of the National Mobile Phone Crime Unit in the UK, DCI Bob Mahoney, phones not protected with a personal identification number (PIN) are worth more money to criminals. The BBC ran an article indicating that millions of phones currently do not use the PIN code, making phones a "gold mine for any thief."
Your phone, like your computer, is an encyclopedia of personal data: logins, credit cards, bank information, email contacts, text and email conversations, photos and more. Whether used directly, pieced together, or used to answer security questions in fraud attempts, there's enough information on your phone to cause you serious issues with fraud, identity theft and even burglary (from your calendar!). In addition, the combination of personal information with contacts in address books and on social media accounts can be used to create effective phishing campaigns against the people you know.
First, read up on how to avoid becoming a mobile robbery victim. Next, follow these instructions to enable to pin on your iPhone or Android device:
For other device types, look up instructions in your manual or online on how to enable your PIN.