“What are the practical uses of calculus, other than calculating the area under curves?”

Among other things, one thing you learn early on in calculus that can be very powerful is Optimization.

If you know how to take a derivative, you can find where the first derivative is equal to zero which can often be a maximum or minimum.

Knowing exactly how to get the maximum or minimum of something can be very useful.

Let’s say you have a certain amount of material to build something and you want the maximum volume. Use optimization.

Let’s say you want to minimize the cost of doing something or making something. Use optimization.