You use geometry for manipulating shapes. You use algebra for doing math with unknown numbers. You use trigonometry for studying triangles, and by extension, waves. So what's calculus for?

Calculus is the study of change.

When you differentiate something, you get the derivative, which represents the rate at which something increases or decreases.

Integration is combining derivatives, or minute changes, into a whole result.

Using these, you can handle changing values in anything: acceleration, electromagnetism, money... calculus can handle them all.

Useful, eh?