Fingerprint sensors will keep the screen awake in Android P, a feature which has been curiously absent from the Android software until now. While it is possible to wake the screen back up with the fingerprint sensor directly after it goes to sleep in Android Oreo and prior, once the screen starts to go dim you'll need to tap the display or some of the hardware keys to keep it from going completely to sleep.
There are obvious benefits to having this feature available, not the least of which is that you can tap the fingerprint sensor instead of the display which will result in not getting smudges on the glass. Since you also won't have to it a hardware key, like the volume buttons for example, you won't have to adjust the volume up or down just to keep the screen from going to sleep then adjust it back. The fingerprint sensor is much more elegant solution to this occurrence. This also means that users won't need to adjust the display timeout length either, so if you prefer to keep it at 15 seconds which is the shortest possible time, you can do just that and simply tap the sensor when you need to reset the timer. Then when you're done using the device you can set it down and the screen won't be left on for a longer period of time.
While this is a feature that's present in Android P, it's also reportedly limited to Android P Developer Preview 2, which is currently the latest version out as Google released it back during the first day of its I/O 2018 developer conference. That being said it should be launched as an available feature with the final build of the software once it goes live later this year, though there is always the possibility that Google could either pull it or expand the support to devices on Oreo as well.