In white-box testing the tester is concentrating on how the software does it. For example, a structural technique may be concerned with exercising loops in the software. These techniques are often used first to assess the amount of testing performed by tests derived from specification-based techniques, i.e. to assess coverage. They are then used to design additional tests with the aim of increasing the test coverage.
Structure-based test design techniques are a good way of generating additional test cases that are different from existing tests. They can help ensure more breadth of testing, in the sense that test cases that achieve 100% coverage in any measure will be exercising all parts of the software from the point of view of the items being covered.
