when things are dark, and you need more light, it dims things.
Exactly the opposite: when things are dark, your pupils dilate and you need less light. Do you turn your smartphone brightness down in bright sunlight?
But brightness isn't the point -- color temperature is. I haven't read the documentation; the software is too simple and easy for that, but the homepage describes what it does. Flux lowers the color temperature at night, which interferes less with sleep. I also find it much more pleasant.