Cell phones are small and expensive. This is why thief targets these gadgets. Small things are easy to misplace or get stolen without the owner noticing. Cell phones are used to store a number of sensitive or even confidential information, e.g. emails, PIN, and credit card details and worse your text messages, imagine if all your information is stolen and another person has access to these. It is highly recommended that one should lock their phone with a password. The finder of the phone will have your handset but not access to all your information.
Also report stolen phones to the nearest police and go through the EMEI blocking process at an orange shop near you.