Otherwise the outer part of the blade would be stalling like crazy. Maybe this reduces noise as well by limiting turbulence.
That's not the reason the blades are twisted.
They are twisted to ensure that lift at the tip is equal to lift at the root.
This is achived by twisting as you say, thus reducing angle of attack and aspect ratio, guaranteeing that lift os proportaional and the same along the lenght of the blade.
If this was not so, the blade would colapse and fold, due to the greater lift present at the tips.