It allows the pitcher to impart spin on the ball. If balls were seamless it would be like trying to curve a raquetball. Although you can put a lot of spin on a raquetball, it doesn't really curve until it strikes the wall, then the spin changes the balls direction. The friction of the seams when released from the pitcher's hand helps curve, or in the case of a knuckle ball, jump through the air. The above is a consequence of a Baseball having seams. The main reason a baseball has seams is that there is no way to 'weld' two pieces of leather together. The rules in baseball state (among other things about the ball) that the covering must be made of leather.
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I am not sure why it has different sized seams but I know why it has seams. The seams help it spin . When you spin it it goes faster. try researching
Because they did not know that seams were important yet. They find out that seams are very useful so they then added seams
The seams on a baseball help the ball curve when a slider or some other breaking pitch is thrown. The seams catch the air and make the ball move when thrown correctly.