Do you want to know who's calling before you answer? Even if it's from a number not in your contact list?
Do you ever want to do reverse number lookups to find out who (a person or company) owns a phone number?
The CallerID app will tell you who (name and full address) is calling, and even display a map of the location. It can even say the name of the incoming caller out loud using text to speech technology. You can add this information to your address book with the push of one button.