1. Lowering brightness is basically lowering contrast; some people can’t read black-on-grey so well. A lot easier to read (bright-)white-on-black, while still not getting so much light exposure. (Have you tried reading a book on your phone in bed at night?)

2. The rise of dark-mode popularity seems to coincide with the availability of OLED screens, where black is a very, very dark color (i.e. what your screen looks like when it’s off) compared to what it is on LCD screens (where it’s just “what the backlight looks like when the LCD is trying its best to block it.”) In these cases, not only is #FFF-on-#000 is seemingly higher contrast than #000-on-#FFF, but also uses far less battery besides.