It has the sensors your iPhone lacks for HealthKit to keep track of your health, fitness, and exercising. It can provide payment through ApplePay without getting your iPhone out of your pocket or purse. You can answer your iPhone or initiate a phone call without getting your iPhone out of your pocket or purse. You can see and respond to messages without getting your iPhone out of your pocket or purse. It can provide Maps directions without getting your phone out of your pocket or purse. You can ask SIRI questions and get answers without getting your iPhone out of your pocket or purse. You can enter a hotel room without a key. Clear airlines without a boarding pass. . . control your AppleTV. . . and a host of other things right on your wrist.
There are thousands of AppleWatch only apps ready to go for the AppleWatch on launch.