You have inadvertently witnessed the use of structure among your objects via interface inheritance and object composition, which allowed objects to possess the ability to obtain new functionality from other objects. In essence, these enabled an object’s power. Continuing with this thought, the weaker the structure, or the flimsier the support, the less empowered the objects.
Consider how reusable a Concrete class is without any inherited interface. The structure that utilizes this object is forced to bind itself to a single implementation and not to that of an interface. Of course, this would be unwise if change ...