I think they’re trying to send a message that international travel to and from Australia should only be done if strictly necessary.

Just for reference I know two people:

1). Someone that before the pandemic would spend, per year, half her time in the US, half the time in Australia. She got to leave Australia for a holiday to the USA without exemption under the “resident ordinarily overseas” and came back when cases started rising in the USA

2) My cousins brother (Aus citizen) who flew here in December, normally resident in Europe, for a holiday of a month and then again, left the country without exemption as they were ordinarily resident overseas.

Both the above situations wouldn’t be possible anymore without getting an exemption, hence, i think the message they’re trying to send is you shouldn’t be travelling overseas unless it is strictly essential