As you get older, your corneas will begin to blur light into dark areas, at that point it is almost impossible to read black text on a white screen.

Dark mode makes it possible to see far smaller things than you can see otherwise. This is particularly important on phones with their tiny screens, or when doing things with lots of details, no matter how big the display.

The default used to be light text on a dark window, for decades before the Mac and Windows came along. There were many reasons for it besides avoiding screen burn.