Why use the standard tests?

Ever since they introduced the testability framework in 2009, Microsoft has been building on its application test collateral. As already pointed out in Chapter 4, The Test Tool, Standard Tests, and Standard Test Libraries, it contains an immense number of tests. These tests cover the whole standard application, from financial management, sales, and purchase, through warehouse and manufacturing, to service management. With every major release or cumulative update, new tests have been, and will continue to be, added to cover new features and recent bug fixes. Years of work we all can profit from. If your code extends the standard application, what will the impact be on it?

You could go about writing your own tests. You could also choose to run the standard tests and see the results. And, of course, in the end, you could do both, as your extension most probably will not only change standard behavior but also add new functionality not covered by any test...