Because I was shocked to find out that…. unit tests are written only to give the team something to do.
They had to develop for one of their client a web application. They had a team of 4-5 people that worked on the project for 2 years and a half. In this period of time, they didn’t written NO unit tests.
After this period of time, the development part ended and monitoring and supported phase started. The team reduce two 2 people that started to make bug fixing, adding small new features. Things like that, normal tasks for this state of a software product.
Because the two members of the team didn’t had enough load a bright ideas came from the ‘God’.
When you don’t have issues or tasks in the queue, start writing unit test.
Of course, unit tests helped them to discover issues in the application. On top of this, they realize that a part of the system cannot be tested because deferent components are too coupled between each other and started to do refactoring.
The good part is that they started in the end to add also unit tests to the system. This is a good thing, especially for that 2 developers. For them, that was the first moment when they realized that Unit Tests can help you, they are not only a waste of time.
On the other hand, even if you don’t cover all your code with unit tests, you should at least cover the most critical part of the system with unit tests.
Also I have a personal RULE that to follow all the time (backend):
When I have an issues or a bug in the system I will NEVER try to fix it until I did not replicate it using a unit test.
Why? When you are able to reproduce a problem in a unit test, this means that you were able to isolate the use case, understand the problem and not the least REPRODUCE it in a controlled environment. This can be hard in a complex system but this is another story.
In conclusion I would like to say only one thing:
Write unit tests starting with development phase.