Training Power was created with a focus on Power Training. It allowed you to monitor your power output and in particular, where you're sitting in your respective Power Zones. Over the years the App has developed and grown to include other metrics and layouts.
Trainging HR was created as a companion App to TrainingPower and to be displayed alongside in an Activity page. TrainingHR had a similar Zonal training focus as TrainingPower, but in this case on the Heart Rate metric.
Radian was developed after I tried a few different watch faces and decided I liked features from some or all of them. I wanted a watch face that had everything I needed.
In The Zone was developed to leverage the new ConnectIQ platform update as well as new device firmware. It incorporates many new features and efficiencies gained with the new ConnectIQ and device firmware and finally brings Weather onto the page.