The most powerful mental models are hierarchical: simple at a high level, but highly precise when you need to zoom in.
Good software APIs should reflect the mental models of domain experts: the syntax should match the most intuitive and most effective way to think about the underlying problem.
A side effect is that good APIs are modular and hierarchical -- they feel simple at a high level, but they are precise & expressive when you need it.
Tools are a reflection of how to think about problems. Good tools are shaped by good mental models, and in turn, they shape the way of thinking of their users.
