This is used by g-s-d power to turn off the display when e.g. locked. A proper implementation will need something like https://github.com/swaywm/wlr-protocols/pull/23 to turn the output off via kms.
See also #28 (closed) for the interaction with org.gnome.SessionManager.Presence. There's also interaction with org.gnome.ScreenSaver since g-s-d power uses this to detect if the ScreenSaver was activated.