Unit tests give us the confidence to change applications, even legacy applications that we didn't write ourselves. To avoid the drudgery of writing a test case for every edge and boundary, we can let JUnit Factory generate a large set of characterization tests for us. If we need to explore an existing code base, characterization tests can act as a learning guide by documenting what the code actually does. If we need to change existing code, characterization tests act as critical "change detectors" that help us avoid unforeseen side effects.

Tests Give Us the Confidence To Change
We often inherit applications that we didn't write. The application works and the users seem happy, at least for now. But, we still need to be ready to maintain the application as soon as the next bug report or enhancement request comes down the pike.