Back in those days, the slingers - soldiers who used the slings - stood behind the spearmen and before the bowmen. They were an important line of attack in the army, causing many casualties. They practiced throwing rocks for long distances and had considerably good aim. Since the blows didn't cause mutilation or blood loss, the Roman writer Vegecius affirmed that the deaths caused by this weapon (by contusion) generated a lot of confusion in the battles.
This weapon was also really useful, according to him, when occupying a stony land, defending a castle, a mountain or an elevation.