While Australia has only had diesel-electric submarines (SSKs) from about 1914 to the present day, these have had very limited discrete Intercontinental speed and Range. They need to indiscreetly show themselves (surface) during 1 to 4 refuels even to get to India or most of Southeast Asia and back. These realities have limited our SSK's roles to defensive or, with a stretch, intercepting shipping in the straits and narrows within the arc to our north.
The speed and range of Virginias and then SSN-AUKUSs will give Australia capabilities of Intercontinental reconnaissance in peacetime and attack in wartime outside or mainly inside the US alliance.
Marles and Albanese make spurious claims about Australian freedom-of-choice sovereignty. However these new Australian SSN capabilities will be laser focused on supporting the US Alliance - specifically China containment, Taiwan defence, Middle Eastern stability, oil/gas shipping lane protection as well as anti-Russian Pacific Fleet roles. Given SSN scarcity in the US Navy, unless Australia promises this support the US will not supply Virginias to Australia.