I think it helps you better understand how data flows through you program. My input is A, and I need to create D. A -> A', then another function makes B, then

I think it helps you better understand how data flows through you program. My input is A, and I need to create D. A -> A', then another function makes B, then another makes C, then another makes D. You can reason about and relatively easily test each of these steps. However, testing in a repl gets quite messy with calls such as h(g(f(x))). In python, you'd need to decorate you functions with an input/output logger.