Why are MLB baseballs rubbed in mud?

Why are MLB baseballs rubbed in mud? Prior to being used in a major or minor league game, all baseballs are rubbed with mud to remove the sheen and give pitchers a firmer grip on the ball.

Prior to being used in a major or minor league game, all baseballs are rubbed with mud to remove the sheen and give pitchers a firmer grip on the ball.