I use cats so that I can write custom effect/wrapper types for e.g. "must happen in database transaction" or "has audit log" or "requires this level of access". (I'd also flag up the free monad, which is like a generic version of the command pattern - you just write your leaf action types and you get a command object for, well, free). Essentially, anything that you might think of using an annotation/AOP pointcut/reflection for, in Scala you can do it with plain old refactor-safe code and have it checked at compile time rather than runtime.