It's used to hold the frequent flyer number in the PNR.
Amadeus requires an active confirmed segment in the PNR in order to put in the frequent flyer number and keep it there. If you put in a frequent flyer number and there's no confirmed segments, it deletes the frequent flyer number. The frequent flyer number is required in order to tell yield or the automated robot (referred to as the 'droid' by the QF call centre staff) what status you are.
QF9094 QZW-QZY is used as it doesn't block up inventory on a real flight, but in theory one could use some SYD-MEL flight that's 11 months away. It's just bad practice to block up inventory on a real flight, in case someone forgets to remove it.
It must be removed before ticketing or the ticketing automated robot will reject the PNR. So if you see that flight still in your booking, after you've provided payment details, then you need to call back and get it removed.
If your PNR goes to manual ticketing, sometimes they blindly will ticket it without removing the flight.