Ancient hunters used slings to kill or stun small prey and while sling stones had the capacity to maim or kill, soldiers often used them simply to distract or harass enemies.  Because archers relied on their range and aim, slingers could be very effective against them.  Slingers didn’t have to approach archers on foot, a deadly prospect, and the stones they hurled into a formation of archers wounded many, killed others and certainly broke their concentration and threw off their aim.