Mostly for ATC, but also for self separation and avoidance.
Underfire, correct me if I'm wrong, but as I understand it Australia (unlike the US) is not implementing ADSB-IN, therefore we will be denied the ability to see other ADSB returns and therefore unable to self separate. When I shell out the money for a new Garmin GTN unit to achieve the required C146a input, I will not have the option of seeing traffic like our American counterparts.
For the RPT sectors, I can see absolute benefit from ADSB. Especially the non radar RPT destinations like Mildura.
However, for a private GA IFR pilot like me who flies below 10,000 ft about 90% of the time, I cannot see any benefit because I will be still mixing with non ADSB VFR traffic and even non transponder RA(Aus), historic and glider traffic.