What's the purpose of this button on my E90 M3?

Hi all, first time posting here (first time posting on any forum i believe )
Anyway, i've had this car for a long time, but never figured out the purpose for this particular button. This is also my first bmw, so please don't laugh too loud if my question is really dumb
I know what the button does (cooler air if roll towards blue, warmer air if roll towards red), but why? If all I want to do is adjusting temperature, why not just turn the knob and adjust the temperature setting?
i searched thru the manual, but don't think that button is even listed