They are part of I think what is Quicksilver's VVS (Variable Vent System). The idea is you can get different bungs with different sized holes. They allow a varrying amounts of exhaust gas to pass out and into the blades causing them to slip slightly on acceleration. This gets the motor up into the power band and the boat going on the plan much faster.
I have no bungs in my Laser 2 and it acts like a turbo. Upto about 2000rpm it is normal then the prop vents slightly from the gases and the revs shoot up and the boat takes off without moving the control lever any further!!
Most boats run pefectly well plugged up and theres no real gain in getting ventted plugs unless you want fas t acceleration for skiing etc.