"Manual testing is often quick and easy and satisfying – you can directly test your application, one can see the results immediately on your screen, and one can interact with the application “for real”, instead of in the sometimes-awkward scripted/mocked mode of unit tests. It’s a very natural instinct.

However, it’s also largely-wasted effort! A manual test only verifies the current state of the code base. As soon as you make a change, you’ve started to invalidate the results. If, however, you take the effort to encode the test in code as an automated test, it continues to be valid indefinitely into the future."

https://blog.nelhage.com/2016/12/...