When the idea was originally conceived, it was during the last mining boom, and when the ADF was battling massive retention problems. The powers-that-be finally conceded that 'Army Presence In the North' (APIN) had its costs (as if nobody saw that coming ...). A posting in Adelaide would provide 'respite' from endless tropical North 'top end' postings, 1 BDE & 3 BDE.
Another way of selling the concept is the realisation that having entire ranges out of action for the duration of the wet season was not conducive to good training; note also the expansion of Cultana (something like a doubling of the size of the range). So 1 BDE units and sub-units will now have year-round access to training areas (and Adelaide's deep water port and defence infrastructure etc), and families have a 'respite' posting location.
For what it's worth, this little black duck thinks the entire APIN concept was a crock, and not worth nearly the cost to families and to the tax payer that has resulted. ANAO calculated the cost of a soldier in Darwin is $15k more to the taxpayer than one in Brisvegas or Sydney. There is also the problem of FBT for provision of a couple of round-trip flights home for each defence family; the ATO deems members of the ADF to be no different to other employees in this regard.
C'mon .... it's no longer 1942. If we're going to retreat into a DOA shell again, then post all the Ronnies & Pussers up to Darwin, Townsville & Derby instead, so they can defend the 'sea-air gap'.
A recent Govt. White Paper (the last one???) recommended "growing" 2 more Inf. battalions so the Aus Army would have 10 battalion sized battlegroups. But, a relatively recent White Paper also suggested that Australia should have a 18 sub navy, which isn't going to happen. So, who knows.