A recipe on the worktop, a map, a sheet of music, a tutorial, something to show someone. One tap in the quick settings and the display stops going dark — for five, ten or thirty minutes, then it goes back to normal on its own.
Because it is the only way an app can stop the display from turning off — and this project found that out the hard way, by building the other thing first and throwing it away.
A wake lock, the API the documentation points you at, costs no permission at all. It also stops working the moment the app leaves the foreground: the system marks it disabled and the screen goes dark at the usual timeout, foreground service and all. The same feature on custom ROMs is not an app — it lives inside SystemUI, a system process whose locks are never disabled — and that privilege cannot be borrowed.
What does work is an invisible window, one pixel wide, that asks to keep the screen on. That is tied to a window being visible rather than to the app being in the foreground, so the system holds the display for us. It draws nothing over what you are doing and it catches no touches. It is one pixel, and it is the whole trick.
Measured on a Galaxy A15 (One UI 6), a Galaxy A20e (Android 11) and a Pixel (Android 17). Every number above, and how it was taken, is written down in the project's notes.