why do I need the sensors for shot tracking if they are not recording any other swing stats? There are lot of other app that you can use on watch.

The sensors are part of the package to register the hit and tell which club you have struck the ball with.
The phone provides the GPS location where you struck the ball. it takes five rounds of recording data before it can suggest which club to hit based on your abilities. the Courses are in a database which you download before your round. All your tendances are recorded for you.