Once you get one, your ABN needs to be provided when you conduct certain forms of formal business activity, which require identification.
For example, your ABN is used for:
identifying your business to other businesses or the Australian Government during trade or compliance activity
registering a business name with ASIC
invoicing and ordering
registering for GST or claiming GST credits
claiming energy grants
registering for PAYG
creating or registering an Australian domain name for your website
processing tax.