Having an ABN can make it easier to keep track of your business transactions for tax purposes and can separate your business activities from your personal activities.
For business with a GST turnover of $75,000 or more, you may have to have an ABN and to be registered for GST.
You will also need to have an ABN and be registered for GST in order to claim GST credits for any GST you have paid for goods or services you have used in your business.
For more information, visit https://www.ato.gov.au/business/