Optus routinely charge first and last month (Same as most rental agreements do FWIW). The idea being you have paid for the first month upfront. If you now default and never give Optus any money again then instead of Optus cutting your service on the 1st day of the second month they have a one month buffer where service can continue and you have time to pay.
So it works both ways. Optus "keep the lights on" if you don't pay on time every month and Optus also aren't out of pocketif you skip town.