The idea is, if the camera OPTIONALLY outputs a noisy picture, you can try to remove the noise yourself on your PC, where there are more options available, and it is generally done better than on the camera (no time constraints).
However, you can't restore the lost detail on a NRed picture.
And yes, I looked at some ISO 800 raws, the noise is pretty strong, but the image somehow looks cleaner than the NR version.