The iPhone firmware's built-in clock function is highly comprehensive. The World Clock itself allows you to pick identifiable cities in all available time zones so that not only will you have your local time on your iPhone displayed, but you will also have a long list of other location times so that you can browse easily. This is perfect for the growing international communities where people may live in London, have their base job location in New York, and have family that lives in Busan, South Korea.
Along with this is an Alarm system that allows you to schedule a whole host of different "wake up" alerts in a way that stands independent. The clock's alarm is a nice addition because it does not have to be tied to a specific event, as it is in the iPhone's Calendar. This leaves two more important, but often looked over, clock functions. The Stopwatch and the Timer are also nice iPhone clock features that can have their own place in your normal iPhone usage.