The API will let developers work directly with a rapidly expanding universe of drivers who are using open hardware to connect to their cars. Open hardware covers a large number of devices that current work with Dash and various other apps, most commonly used to servicing and performance tuning.
It is hard to know how Developers might use the API and that is part of the point. The “Internet of Cars” might result in any number of applications. For example changes in “Braking” might be used to acquire realtime road conditions. Or engine performance data could be used to trigger preventative maintenance before more expensive failures.