I'm not real knowedgable on the manufacturing methods of Cannon balls, but they were cast in molds. However the molds were fitted well, and the seams & sprue were filed/ground smooth, then the balls were run through ring gages to determime if they would be accepted by the government.

Most cannonballs that I have seen did not have any evidence of the parting seam/sprue visable. Many now exhibit pitting such as you indicate is present on the one you found.

As to other possible iron ball uses, they could be used in grinding/tumbling operations, demolition devices, rollers and other uses, even decorative. And of course-it may be exactly what you suspect-a cannonball!