This could either be done at start-up, or while the Gio app is running.
Being fullscreen can be useful for:
In my case, right now I want it for the end-to-end tests, just so that I don't have to deal with window decorations or status bars, like on Android and Windows.
I should also note that some environments allow making a window full-screen from the outside. For example, many Windows and Linux environments allow you to do that with any window by default. However, other environments are not like that. For example, Android requires the app to enable "lean back" or "immersive" modes: https://developer.android.com/training/system-ui/immersive
https://gioui.org/commit/238dd1aa implements fullscreen support (most of) the desktop OSes.