Australia has an entry for a major city per state not because someone decided it was a good idea, but for the same reason Boise got one - they were the largest city in a jurisdiction that sometime after 1970 changed its timezone so it didn't match anyone else which necessitated adding an entry in order to record the unique timezone rules for them.

For instance, both Sydney and Melbourne both have entries because Victoria and New South Wales have started/ended daylight savings time in various years since 1970 on different dates like in 1990 when in Victoria it ended on March 18 while in New South Wales it ended on March 4.

It's also why Broken Hill has an entry despite being in New South Wales, they're in their own timezone.