As a system grows more complex, the separation of responsibility between APIs and between binaries becomes increasingly important. This is a direct analogy to object-oriented class design: just as it is understood that it is poor practice to write a "grab bag" class that contains unrelated functions, it is also poor practice to create and put into production a "util" or "misc" binary. A well-designed distributed system consists of collaborators, each of which has a clear and well-scoped purpose.
—— Google SRE Book, Chapter 9