Object-oriented programming provides encapsulation: an external interface to an object in terms of messages is defined, but the internal implementation of the object is hidden.
Modularity: Objects are often a good way to think about the application domain.
Modifiability: The internal implementation of an object can be changed without modifying any other programs, as long as the external interface is maintained.
Expandability: New kinds of objects can be added to an existing system, as long as they present the same interface as existing objects.