Why auto brightness have the brightness adjust bar?

[acsurfer]

I am not sure how does auto brightness works. I ticked the [auto brightness] function, and I can adjust the brightness level at the brightness bar.

If auto brightness means the phone is going to adjust the brightness automatically depending on the phone sensor, what is the point of adjusting the brightness bar? I am using Samsung S4

 

[raptir]

TouchWiz, the modified Android skin that your Galaxy S4 runs, includes a brightness bar in the notification drawer even if Auto Brightness is enabled. Adjusting the brightness using the bar will disable auto brightness, and you will need to re-enable it if you want it back.