It was a very common weapon in Ancient and even Medieval times but Im curious about the Weapons Effectiveness in a time where pretty much everyone had a shield and some light armor like Leather that would have provided some protection against flying rocks.

How "Deadly" was the Sling and its Missiles? Was the purpose to inflict casualties with withering fire? Perhaps skirmish,slow down or Distract an opposing group?

I Imagine many skirmishers, either as individuals or in groups would pelt an enemy force yards away and have their missiles similar bounce off a row of Shields, maybe strike a helmet if its lucky.