BYO (Bring Your Own) is very common in Australia for a number of reasons:
1. The BYO license is cheaper than a "Full License"
2. It may cost too much for a restaurant to establish their own cellar (space and stock)
3. The restaurant may not have the correct expertise in selecting or offering wine
4. It is very ingrained in the culture
Some restaurants have their own cellar but also offer BYO. Corkage is either per person or bottle so it may take some calculating what is cheaper.