We would like to reuse as much of a design as possible in this or in other projects. Abstraction helps reveal the structure. But we would still not be able to reuse a design unless we understood how various objects depended on one another. For example, it is much more economical to think about number in the abstract than to keep all the operations for all the types of numbers straight. That the integers and real numbers are both numbers allows us to inherit information and processes from their common parents.
A good design will make these relationship evident.