I can't answer your question in terms of pragmatic applications. Based on skimming this book and similar resources posted on HN, my -- naive, I am sure -- understanding is that category theory unifies other theories and serves as a glue.
One example from the linked book, "Remember that we said that programming types (classes) are somewhat similar to sets, and programming methods are somewhat similar to functions between sets, but they are not exactly identical? A formal connection between the two can be made via category theory."
One example from the linked book, "Remember that we said that programming types (classes) are somewhat similar to sets, and programming methods are somewhat similar to functions between sets, but they are not exactly identical? A formal connection between the two can be made via category theory."