Object-oriented programs use inheritance and virtual functions to achieve powerful abstractions and good modularity. By postponing until runtime the decision regarding which specific function will be called, polymorphism promotes binary code reuse and extensibility. The runtime system automatically dispatches virtual member functions to the appropriate derived object, allowing you to implement complex behavior in terms of polymorphic primitives.
You can find this kind of paragraph in any book teaching object-oriented techniques. The reason it is repeated here is to contrast the nice state of affairs in “steady mode” with the unpleasant “initialization mode” situation in which you must create objects in a polymorphic way.