And even more meaningfully for tool authors, the plumbing is very long-lived and general-purpose, with basic objectives like not leaking and not breaking down, while the porcelain is rather easily replaceable, possibly specialized, "smaller" but more complex than plumbing, with meaningful user interface (in Git's case, specifically, designing commands that are unlikely to do harm, easy to understand and efficient after they have been learned) and significant dependence on tastes and environments.