Why do most of the settings that are suggested for TVs specify turning the backlight and brightness down to the fifty percent. or so, range?
It's not so much the brightness itself, it's how the brightness is used within the metadata to create what pro calibrator, DNice has termed as "Spectral Hilights". These hilights dramatically improve the PQ details that you may not otherwise see - it has not much to do with Menu brightness setting.
Combined with other features (Ten Bit Panel/QDot/WCG, it simply creates a sigificant improved PQ over previous panels without HDR capability by most that have witnessed it many stating it's a game changer but it's not so simplistic as a Brightness Setting.
Joe Kane, Scott Wilkerson, Professional AV persons could explain it much better than I. To me the Spectral Hilights using the NITS brings out details otherwise hidden by DSE, Black Crush or creating depth that makes it similar to glasses free 3D.
I've observed something akin to this where the CR was so great on an SXRD I owned ten years ago that it did exactly what I see HDR viewers report and amazing WoW factor I've not seen since on any LCD/Plasma.