Modern smartphones are very intelligent in how they handle screen brightness. That's because you need the screen at a certain brightness to see, but if it's too bright, it just burns through battery life.
That's why your screen is supposed to adjust automatically based on the ambient lighting, how long the screen has been active and what you're doing. If you want, you can override the screen brightness in Android, but then you lose the automated adjustment.
Lux Auto Brightness gives you control over the screen brightness and leaves the intelligence in place. You can customize everything about the brightness, including how it reacts to ambient light and more.
Even better, you can use Lux to adjust the screen color temperature. This lets you set the tint to a reddish color at night, which reduces eye strain and helps you sleep better - similar to the F.lux program for computers.