Quoted from fugus:

I think pops are mainly used to let the player know a ball is in play and get ready for it to come down the playfield. Thats especially when the ball is launched.
I've heard that pops are there to randomise the ball direction, though the pops only usually feed out in one or two exits which is hardy random.
A spinning disc does the same job. I think f14 firepower and rob zombie make good use of pops. If your shooting is off your going to lose ball control big time.