I don't know, to be honest. They use them at festivals overseas to divide the crowd into smaller sections so they can't rush the front and cause a crush. The Big Day Out started using them after Fred killed that woman.
Having been to eight of the ten BDO's in Sydney,
I actually think it made it more dangerous at the BDO when they did that because all it did was move the crush from the front of the stage over to one side of the stage because the entrance to get into the front mosh-pit was through a gate about 2 foot wide and then through a narrow corridor (about 5 feet wide!) before opening out into the front area>when System of a down played about 5,000 people are trying to crush to get through this tiny little gate(and no i wasn't one of them.i was watching from the grandstand) to get into the front area.In the end about 1000 people all realised at the same time that if they all jumped over the outer(2nd) barrier(instead of being herded like cattle through the gate) then security wouldn't be able to stop all of them and the vast majority of these 1000 made it into the front area thus negating the efects of trying to limit the amout of people in the front area between the front barrier and the 2nd barrier