Calculating taxes is handy for a variety of reasons. The first is that there is gross income and net income. Net income is what actually appears in your bank account every pay cycle. Jobs are often quoted in gross salary in Australia. So for example, you might have a $100,000 salary, but receive less than that because of taxation. This is important when it comes time to figure out your budget, how much you can afford with rent, the mortgage, and more.
The second is that you’ll quickly realise if you’re being taxed too much or too little. The third is that if you receive a bonus or other work-related windfall, such as leave loading, you can work out ahead of time how much you might be taxed on that pay cycle.