The purpose of testing

When starting to learn about test-driven development, many developers struggle with the question: "Why are we doing this?" This is also reflected in the tests they write. They write tests to verify the framework they're using, or for simple trivial code. They also write brittle tests or tests that are testing too much. They have not reflected on why they're testing and often only do it because they've been told to, the worst kind of motivation.

The value of testing is shown in the following image:

The original illustration comes from a talk by Martin Fowler on refactoring. The title is Why refactor? and the same applies to testing. The value of testing comes not from quality, clean code, professionalism, or that it is the right thing. The value is economics. You write tests in order to save money. Bad programming will lead to bugs in your software, which can have the following consequences:

We need to avoid bugs in order to avoid unnecessary and hard-to-predict costs. By adding testing to our process, we create predictability and reduce the risk to software development projects.

An estimation of 20 million Americans experienced the broken ObamaCare site, seriously hurting the reputation of software developers worldwide. By writing tests for our code, we will achieve higher quality, cleaner code, and maintain a higher level of professionalism, but what it eventually boils down to is that the code we write will have greater value. Tested code will:

All of these points of interest lead to predictability, a precious thing in software development.