Because you are charged this if you dont setup direct debit.
From the plans page and CIS document:
Non-direct debit fee:A $2.20 fee will be charged each month if you choose not to pay your bill by direct debit. To set-up direct debit go to optus.com.au/myaccount Exception: For payments made by credit, debit, charge or prepaid cards, the non-direct debit fee will not apply, but payment processing fees may apply
How to setup direct debit:
https://www.optus.com.au/for-you/support/answer?id=9606