When you turn down the brightness on a light theme, you're also turning down all contrast just because the background is emitting too much light.

A dark theme has the simple solution of turning off the "lightbulb" of a white background. Now the only light is produced by they actual content, so the contrast can remain high.

Though, I also am annoyed that I can't turn down the brightness on my macbook/iphone even more (e.g. in a pitch black room). That is one thing that has indeed forced me to use a dark theme when I wouldn't necessarily have thought of it.

For some devices/screens at night, the lowest possible brightness setting ends up being what I would have guessed was maximum brightness.

I also experienced recently where lubuntu was missing several intervals between middle and off that were in default Ubuntu.

I usually use redshift to decrease the brightness even further than what my laptop allows me, as well as adjust the colour temperature. It's kind of annoying that the lowest brightness is still too high in the dark on almost all laptops, but perhaps there's a technical reason for that.