It's come from people attempting to keep the pre 1990 " radio arranged separation" techniques in the twenty first century.
In the old system CTAFs did not exist- all calls at non tower aerodromes were on the FS frequency allocated to the area where the aerodrome existed.
It worked pretty well but cost a lot of money to have FSOs monitoring all these calls.
Aussie pilots should note- don't try making announcements on ATC frequencies in other countries - it's not allowed due to safety reasons. All communication must be directed to the Controller.
No doubt after an unnecessary accident or serious incident it will be prohibited here too.