When calculus is taught in a classroom setting, it is often taught as "pure" mathematical content; it is a mathematical technique for the sake of mathematics that has no specific practical purpose in such a "pure" setting, besides developing logic and reasoning skills.
Applied mathematics, and thus applied calculus, borrows from the language of "pure" mathematics, but has a practical and definite purpose.
Scientists and engineers in particular use calculus and other mathematical techniques to solve applied problems relative to their field. For example, applied algebraic techniques tell us that a car will travel a certain distance if traveling at a constant speed in a set amount of time. Applied calculus could tell us how far a car will travel if traveling at an inconstant (i.e. accounting for acceleration) speed in a set amount of time.