Refer to figure.
The blade tip is rotating much faster than the root, so the airflow mass across the tip is much greater than the airflow mass across the root. Assuming a blade of constant chord, the amount of lift/thrust produced would increase rapidly with distance from the rotor hub.

The large amount of lift/thrust is undesirable at the blade tip, because it increases the blade tip vortices.

The solution is to progressively reduce the blade pitch angle (and as a consequence the angle of attack) as distance from the blade root to tip increases, giving a more even distribution of lift/thrust spanwise. This is achieved by designing a certain amount of twist into the blade, known as "Washout".

A blade with carefully designed Washout can make a huge difference in evening out the lift production along its span.

Moreover, Washout relieves the large bending stresses in the rotor blade that would, otherwise, be created by the large amount of lift produced close to the blade tip.