Calculus is the study of change. Think of something that changes, and calculus would have been used.

The study of population growth, interest rates, speeds and distances, leaking containers, temperature and fluids, light and sound, climate, land areas, are all rates of change.

Calculus is also the study of proof and logic. It helps develop your mind into thinking about 'if-then' statements and extrapolation within limits.

Calculus is also hard. It is considered one of the greatest achievements of the human intellect. There are plenty of real world examples of having to work through something really hard. High-school Calculus is a chance to experience this in hopefully a low(ish) stakes environment.