The mass fire tactics used with muskets with ball shot were developed in part because of the relative inaccuracy of a single shot from the weapons. The development of the minie ball made each individual capable of hitting his target over 100 yards away. So you go from people shooting at masses of people and hoping that they might hit something to masses of people who all can hit within around 12 inches of what they are aiming at every time.

Before the minie ball each grouping of men was like a giant shotgun with each man shooting one of the balls in the hopes that enough would hit to cause some damage. Now each SOLDIER is capable of doing the damage that it might have taken up to 10 (maybe more depending on the range) soldiers to inflict.

I found the following article to be a decent synopsis of the minie ball's effect. http://www.rossfordschools.org/vnews/display.v/ART/4ea1872c97a49