It is difficult to come up with concrete examples of category theory because it is too high up the abstraction hierarchy. It normally requires stepping through

It is difficult to come up with concrete examples of category theory because it is too high up the abstraction hierarchy. It normally requires stepping through several layers of abstraction to reach something concrete. Monads are one specific application of category theory. Of course monads in and of themselves are still very abstract, so we need to drill down to another level of concreteness by taking a look at the "Maybe", "Either", or "IO" monads. This is still very abstract so we can drill down to another level to "does the data exist", "is there an error", and "how do I handle IO"? Category theory is a toolbox of mental representations useful for reasoning about a particular problem set (designing formal systems that let you design formal systems). In other words, category theory is an abstract concept for solving a slightly less abstract set of problems. Asking for concrete examples doesn't really make sense because it is too far up the abstraction hierarchy. To make the case for the importance of category theory, it is necessary to present the entire abstraction hierarchy across a number of different real world domains. This is not something academia is good at because it tends to isolate and subdivide work into increasingly specialized fields.