In a recent survey of 115 mobile users asking what mode they generally have their mobile device in, roughly 1/3 said dark mode, 1/3 said light mode, and 1/3 said a combination of both.
Reducing eye strain is the most common reason users with normal vision mention for using dark mode. As one research participant put it, 'My eyes have always been very sensitive to bright lights. So ideally, I use dark mode on everything I can. [...] I don't think I can go back to normal. [I have] a lot less eye strain. I don't think I'm getting headaches as much ...'
Dark mode does slightly reduce the amount of light being taken in by the retina and might feel easier on the eyes during a single session - particularly in dark environments.