Yes, but it's not the same as turning the display off. The GPU is still outputting signal to the Macbook Pro screen.

Wow, I hadn't considered that. It might explain why I get terrible battery life when playing movies on a TV over HDMI with the screen "turned off" by dimming it down to zero. Thanks for the hint.

Along with the battery suck, it also generates more heat as the GPU has to output 2 signals.

what the.. why? The screen is unusable without the backlight.

Perhaps latency? Suppose it takes 0.5-1 second to switch between "Off" and "Lowest brightness" when turning On/Off the GPU, that could create a bad experience when every other brightness change is instantaneous. Though it is a great power optimization idea, if the (unknown to me) downsides aren't too bad!