This. Also, when in a dark room like if you are laying in bed, don't you want less light coming out of your phone and it not to be like shining a flashlight in your eyes. Dark mode helps there especially if you have an OLED screen so that the text is the only light coming out.
That said, I've heard from many people that they struggle to read light-on-dark. So it's best to have both options, especially for a site like HN where it doesn't require a huge design effort.