Touch ID was designed to allow us to further secure our iDevices using a fingerprint for unlocking along with a security code, iOS 8 offering the possibility to secure information from applications using the same system, but also to access a system called Reachability. What you probably didn't know is that touching Touch ID when the screen has dimmed before it automatically locks will trigger a reset of that timer, with the brightness returning to its original value to allow continued use of our terminals.
The functionality isn't terribly important, but it replaces the need to press the Home button or press the screen to keep it open to still see the images present on its screen. Apple hasn't provided details on this functionality, and probably never will, so learn about it now and you'll likely use it in the future.
