FAQ 7.02 What are the benefits of proper inheritance?

Substitutability and extensibility.

Substitutability: An object of a properly derived class can be freely and safely substituted for an object of its base class. For example, suppose a user defines a function sample(Base& b). If class Derived properly inherits from class Base, sample(Base& b) will work correctly when an object of class Derived is passed instead of an object of class Base. In contrast, there is no guarantee that sample(Base& b) will work correctly when it is passed an object of a class that was produced by improper inheritance.

Extensibility: The properly derived class can be freely and safely used in place of its base class even if the properly derived class is created months or years after the user code was defined. It's easy to extend a system's functionality when you add properly derived classes that contain enhanced functionality. These guarantees cannot be made when improper inheritance is used.

Note that these guarantees work only when the user code relies on the specification of the base class rather than the (possibly more specific) implementation of the base class (see FAQ 6.02).