Why do I need to sign a contract

The VBA (Victorian Building Authority) and Consumer Affairs Victoria confirms by law, you must have a written domestic building contract for work valued more than $10,000

https://www.vba.vic.gov.au/consumers/home-renovation-essentials/building-contracts#:~:text=If%20you%20are%20engaging%20a,a%20major%20domestic%20building%20contract.

https://www.consumer.vic.gov.au/housing/building-and-renovating/plan-and-manage-your-building-project/contracts