Reusability

The Power of Reuse is:

One of the most important features of object-oriented programming is the ability to modify existing solution to solve new problems. If a particular kind of problem has been solved using the OOP approach, a similar but slightly different problem can usually be solved by making some changes in the object-message protocal that already exist.
Most of the time, this requires adding new messages. Other cases may require adding new objects and new messages to which those objects respond.

Reusability is probably the most important and the strongest feature of Smalltalk. Although procedural languages can be reuse too, Smalltalk makes programming for reuse much easier.