If you wanted to get segment credit for LHR-DXB and then DXB-SYD you need to take different flight numbers or dates. At least with QF you stay on the same plane I've had single flight numbers where there was a significant change of equipment (2 aisle to single aisle) at the intermediate stop, and once even missed the "connection" as inbound was delayed and outbound left before the same flight number landed! They had to change the flight number for the 2nd leg that day, and I was stuck overnight -- not on QF though.

The primary reason airlines do this is to get higher GDS position in reservation systems as this shows before any connection with different flight numbers. Improves seat sales significantly (I'm told over 10% more on average). QF does it for historical reasons as much as anything, they have always had a single flight number LHR-SYD and LHR-MEL going back to 707 days and maybe earlier when there were lots of stops not just 1.