Category theory has applications in theoretical computer science and obviously is central to abstract mathematics. I have heard that it also has direct practical applications in programming and software development.

What type of programming is practical category theory necessary for? What do programmers use category theory to accomplish?

Please note my use of "necessary" and "require" in this post. I realize that in some sense most programmers will benefit from having experience in different types of theories, but I am looking for direct applications where the usage of category theory is essential, i.e. if you didn't know category theory, you probably couldn't do it.

Also, I'd like to clarify that by "what type of programming," I am hoping less for a broad answer like "functional programming," and more for specific applications like "writing bank software" or "making operating systems."