It complements regression testing based on commercial test suites and tests locally developed during product development and deployment.
Differential testing requires that two or more comparable systems be available to the tester.
These systems are presented with an exhaustive series of mechanically generated test cases. Implementing differential testing is an interesting technical problem. Getting it into use is an even more interesting social challenge. A working prototype for testing C compilers is available on the web. As a practical matter, quality is not a question of correctness, but rather of how many bugs are fixed and how few are introduced in the ongoing development process.