From a policy perspective, public broadcasting is a form of state intervention in the operation of a market. As for all policy interventions, two fundamental conditions need to be met for the intervention to be justified. The first is that the market left to its own devices fails to produce an efficient outcome. The ABC Charter is not of much help on this. There is nothing unique about these functions and why a public broadcaster is necessary to perform them is not explained.
Historical developments are a little more helpful.
From them we learn that initially, public broadcasting was largely justified by a shortage of broadcasting frequencies. The concern was that a medium with a powerful potential influence on public opinion could not be entrusted exclusively to a few individuals. It was also intended as a correction for the limited programming diversity that usually results from a purely commercial system in a limited channel market. Some market failure arguments can also me made in relation to cultural and educational benefits that arise as externalities to the consumption of programs. Of these justifications, the one based on spectrum shortage has largely evaporated.