The angle of attack is the angle between the blade
chord line and the relative airflow. Trouble is, the
relative airflow is different along the blade. The blade is rotating, so the
bit nearest the hub has a small speed at right angles to the way the aircraft
is going but the bit near the tip is moving rapidly across the aircraft flight path.
To put real numbers on to this, a 10 ft blade doing
3000rpm will have a tip speed of about 300mph, so if the aircraft is going
forward at 300mph the relative airflow is hitting the tip at 45deg off the
aircraft centre line.
Thus, if you want the tip to have the same angle of
attack as the root you have to twist the blade by 45deg along its length.
The speed past the tip will now be more like 440mph in total, so if the angle
of attack and the chord length were the same you would get a lot more lift
(thrust) out toward the tip.
But the only reason why you might not want the extra
thrust is because it might bend the blade, and, if that worries you, you can
taper the blade, reducing the chord length.
Blade design is always a compromise, but if you want best efficiency (least
fuel consumption) in the cruise you will twist the blade so all sections reach
their most efficient angle of attack at the same time.