At differing environmental brightness levels, you should set the slider to what you'd like. The phone remembers and matches the light level with your brightness choice.
So basically it requires a bit of programming initially for more accurate automation of brightness in the long run.
Mine is hit or miss. When I first got iOS 6 it would brighten and dim automatically. I wasn't used to this, it would brighten but not dim, so I would just turn off then screen and back on. Now it's doing the same thing.