hindy111 pretty much nailed it.
This will be a 24/7 airport which suits Euro/American time zone arrivals/departures far better. It will also be, as h111 said, a major boost for freight transport and will have all the benefits he stated - less road transport on clogged local and arterial roads around Kingsford Smith, as well as having distribution of said cargo distributed from the industrial areas of the Aerotropolis. The airport will also be directly connected to Sydney's motorway system via the new M12, so road freight will be able to be distributed north, south, east and west beyond Sydney without using any local roads.
A new 23 km driverless metro line will connect to St Mary's station so domestic and international travelers will have a decent public transport option to the city.
Certainly, low cost airlines will use this airport. Some high cost airlines have also already committed to using it as a secondary off load point as well.
Remember, Sydney AP was pushing capacity before Covid-19, and has been for years. It's not just landing spots, but gate capacity where Kingsford Smith falls short. It's why Heathrow for example, can land planes every minute, and Sydney can't.
You might want to fly in and out of Mascot because you live 5 minutes away.
There's 2 million people in western Sydney who'd probably rather fly out of Nancy Bird Walton International Airport than travel an hour to Mascot.
Your above comment is both ignorant and snobby.
You are confusing Leppington with Luddenham. And the new airport
won't even be at Luddenham.
It's more Badgery's Creek and Bringelly.
And they've been "turning the soil" for over 12 months now. Much progress has been made.
I'd suggest you peruse this document.
https://majorprojects.planningportal.nsw.gov.au/prweb/PRRestService/mp/01/getContent?AttachRef=SSI-10051!20201019T004448.159 GMT
This is worth a watch as well.