Exactly. In the example that I gave, the design described will handle complex edge cases such as a part time employee working for multiple companies. And will

Exactly. In the example that I gave, the design described will handle complex edge cases such as a part time employee working for multiple companies. And will do so without programmers having to think through the whole system at all points. Independence of modules has no importance in a codebase that ships as a whole. But modularity does.