Yes mostly to avoid the quarantine, but also because there's a good chance you'll be able to enter queensland immediately when they hit a 90% vaccination rate whereas you'll probably need to complete 14 days of quarantine regardless of when they hit 90%.
On the other hand, covid case rates are exploding right now and the Queensland government has always been over cautious. At this point I think the cat's out of the bag and closing the borders again would be a politically very silly thing to do, but there's always that chance. So the safest option is still ultimately to enter Queensland directly and go into quarantine.
I still think entering via Sydney or Melbourne is better though. 14 days of quarantine is no joke--I went through 3 days of poorly enforced quarantine, and even that got old really fast.