Apple doesn't allow developers to adjust the system brightness as is done in Settings. We can only adjust "session brightness", which means that it reverts back to whatever is in Settings when the device is locked.
This doesn't work in all situations, but it's surprisingly nice most of the time. If you adjust the brightness down while reading in bed at night, you can just lock the device and won't have to turn the brightness up in the morning.
If Apple ever gives developers deeper access to the brightness setting, we'll definitely add support for it, but this is the best we can do right now.