As I understand it, first-to-use ownership of a mark trumps first-to-file. If that is true, why spend money on getting a trademark for your business name?
Because a Registered trade mark give statutory rights while an unregistered one only gives common law rights
Enforcing an Unregistered Trade Mark
An unregistered trade mark has limitations in the level of protection it can provide to its owner. For an unregistered trade mark, any right to ownership of the trade mark needs to be proven. It is easier to confirm your enforceable rights to exclusively use a registered trade mark. This is because your rights immediately arise once it is registered.
According to the common law, a substantial level of evidence will be needed to demonstrate an unregistered trade mark’s use. The owner of an unregistered trade mark often needs to prove that a reputation has been established, and their level of protection will only correspond to the area where they have established that reputation. A registered trade mark, on the other hand, provides protection Australia-wide.
For example, in my business I would have no trouble proving a reputation for my goods and services is Sydney. I would have no chance of proving it in Melbourne or Brisbane or even in Wollongong or Newcastle so anyone can use my common law trade marks for the same goods and services outside Sydney and I couldn't stop them.
A trademark shows the court that you have created and registered something if you have to enforce it through the courts.
If you went to court just saying a trademark is yours, anyone could do it for any trademark.