I have an eye condition, not sure what it's called, but bright lights really hurt me. Black backgrounds are just natural for me to use because they don't hurt. It really needs a proper color scheme on top of it though. Simply changing the background results in something just as bad.
Its easier readable.
Reduced distractions. Lots of apps and web sites still don’t support full dark mode — come on, it’s 2024! For those I just turn my monitor off.