To take Australia?
To further their ambitions, including Australia.
You talk like China has no plans or future wherewithall. You need to think long-term.
Recall where China was when Nixon opened it up a scant 50 years ago, having a per capita GDP slightly more than $100. A true 3rd world country with limited wealth.
The US is heading downward with limited acumen of the future while China is heading upward with truly massive potential on the horizon.