Why is calculus important?
Since you listed a textbook you might be looking for a specific answer. In general, calculus has many practical applications. It is used in business in project management and finance. It is also useful in engineering and medicine. We would not have many of our innovations if it were not for this important type of math!
Calculus is the math of approximations and change. It has many practical application is business, physics, and engineering, as well as other STEM (science, technology, engineering, mathematics) careers.
For instance, the marginal cost, profit, and revenue functions are the derivatives of the actual cost, profit, and revenue functions. And that's just one example of calculus!