Why would they go to the great expense, time, and difficulty of carving a stone into a sphere? That wouldn’t seem to make any sense. Just heave raw stones as fast as you can collect them.
The Romans used artillery (the word antedates gunpowder, btw) of similar kind as siege equipment against the hill forts in Britain, more than 1000 years before this Edinburgh siege, and they didn't bother to do much shaping when they were hurling rocks. :^)
Ballistic repeatability. Semi-spherical, equal weights, it flies about the same distance each shot.
Random stones would have random flight paths. Consistency makes it easier to hit the same spot over and over.
The damage would be more after the landing
Accuracy at range = firepower overmatch.
The stone spheres were more uniform in trajectory than odd-shaped rocks, which is important when you’re trying to peck, peck, peck a breach through a castle wall. You have to keep hitting the same limited area time after time to get results.
Better ammo means a shorter siege and less money paid to the troops to keep them on the job. And if the soldiers see you’re making progress on the wall, they’ll stick around in anticipation of the ravishing and pillaging to come.