Not only is it a functional feature helping with strain on the eyes, but from a branding standpoint, it just elevates the perceived value of the brand. Users are paying a premium for your service, and a well-designed dark mode would give the brand a more premium feel. I'm willing to bet that if you made a really nice-looking dark mode and only made it available to premium users, you'd get more conversions.
Not only is it a functional feature helping with strain on the eyes, but from a branding standpoint, it just elevates the perceived value of the brand. Users are paying a premium for your service, and a well-designed dark mode would give the brand a more premium feel. I'm willing to bet that if you made a really nice-looking dark mode and only made it available to premium users, you'd get more conversions.