There arguably is no main use case for category theory, although you could talk about its initial roots in fields like algebraic geometry and topology. But the

There arguably is no main use case for category theory, although you could talk about its initial roots in fields like algebraic geometry and topology. But the bottom line is that it's an incredibly general system which has meaningful connections to all sorts of other systems in math, computation, logic, and more. As for an intro essay, to a large extent a resource like this one is intended for people who already know they're interested in the subject. If you're not currently part of the target audience and you'd like to learn more, a brief search will turn up a plethora of resources.