In my own experience, many devices either: * do not offer granular enough control of the brightness on the low end (the backlight is either kind of dim, or nea

In my own experience, many devices either: * do not offer granular enough control of the brightness on the low end (the backlight is either kind of dim, or nearly/completely off, with no in-between setting), or they * do not have enough contrast at low brightness to read easily. In either case, the display is more readable by having the backlight on bright, and darkening most of the display (by changing the background color). Of course, this does not apply to all displays, but every display has pretty fine control its total light output by changing the colors it is displaying.