Why do we learn languages at school that most of us will never remember, be fluent in or use (coming from Australian education background)?

The fact that you are Australian is significant here.

Foreign languages are taught in school because foreign languages have been decided to be useful to a country’s citizens. They can be useful practically, or they can be useful culturally.

Classical languages were initially taught because they are useful practically as well as culturally. Latin was the language the European elite communicated in internationally, and classical Greek is where Latin got its culture from. When the practical utility fell away, classical languages became more of a niche, but they were still felt essential to the cultural grounding of the elite. When the culture shifted away from that, classical languages became even more niche.

There are only a few countries in which a majority of citizens don’t need to learn a foreign language for very practical reasons. And many of those countries are English speaking. In such countries, you learn a foreign language in school either because a minority still will finds it useful, or to contribute to the cultural grounding of the citizenry.

In the US, teaching Spanish makes sense for practical reasons, because a minority of non-Latinos clearly will still find it useful, whether south or north of the border. The same applies in Australia, with teaching Japanese or Indonesian or Mandarin.

The same kind of applies to Britain with the teaching of French and German. There is a bit of cultural grounding going on there, as well, given the importance of French and German culture. But those really are the two languages an educated Briton was most likely to run into.

French and German were taught in Australia, because French and German were taught in Britain. I am grateful that I was taught French and German, but I concede that the priority given to French and German until 20 years ago in Australian education was an anachronism.