The main reason to do this is the undersupply of units that has been going on for at least 3 yrs now. When I came 6 yrs ago it was easier and I had to supply my paperwork (i.e. employment contract as a reference) but no extra rent payments. In those days rental supply was about 5% vacancy (2003). Now in many places rental vacancy is < 3% and this has led to alot of applications for very few places.
No State is caught up or has a plan that shows it will catch up such that rental vacancy rates will become better (> 5% again).
This means undersupply will continue for the coming years and affect all renters
Rents will continue to grow in coming years (level in a bad economy, increasing in good economy years).
Have all your paperwork ready to lodge on the spot. Most agents have their application form available online.
To put yourself ahead in the queue, offer to pay a few months rent cash upfront. That is what I had to do when I first moved back to Australia and didn't have any references or a job.
You have to be organised if you want to secure a place quickly.