Comment by literalAardvark

It is significantly more efficient for oled displays, as off oleds don't use power. It also causes burn in on a smaller part of the display which is usually good (but this could end up being a disadvantage over time as the burn in contrast is higher).

It's also more efficient for led matrix backlights.

Edit: sorry, realized this is misleading: my testing was with light vs dark, not something like dark grey vs 00 black