This does two things; first, it increases the accuracy of the projectile by eliminating the random drift due to the Magnus effect, and second, it allows a longer, heavier bullet to be fired from the same caliber barrel, increasing range and power (see external ballistics).
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However, spherical bullets do tend to rotate randomly during flight, and the Magnus effect means that even a relatively smooth sphere will curve when rotating on any axis not parallel to the direction of travel (see knuckleball for an example of intentional random tumbling.)