* 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.
f.lux nicely takes away only the blue light but this isn't always enough for the bright backgrounds of modern homepages and text editors.
[1] https://github.com/m-khvoinitsky/dark-background-light-text-...