It's all about selective education.
If not, the good public schools like Balwyn, Glen Waverley, Brentwood etc. No way I'm sending my kid to a private school even if I have the $$$. It's simply not worth it :X
The problem with private schools is that I find too many parents think it is a quick fix to make their son or daughter a good person. I see too many parents under the impression that their kids are fine, yet in reality they take drugs, smoke, sleep around and whatnot. Is that a private school thing? No, it happens anywhere, so spending another 20000 won't better your chances THAT much. A good education generally begins at parenting. If you raise a non-selfish, studious, friendly kid, it is far less likely that they will fall prey to the wrong hands and go wrong.
I think reasons for sending kids to private schools may include scholarship acceptance, religion, sporting/music facilities, or any other facility at the school which will DIRECTLY enable your child to succeed in whatever they are interested in. Nevertheless, the number one reason that most parents send their kids to private schools is so that they can feel as if they have contributed to something positive for their child.