Unit and integration testing not only make code less buggy, but lead to a calm state of mind. Bugs show themselves quickly, along with which method is failing,

Unit and integration testing not only make code less buggy, but lead to a calm state of mind. Bugs show themselves quickly, along with which method is failing, so the fixes are fast and effortless when you break stuff. Massive refactoring doesn't scare you, and you sleep soundly with no fear of hidden breakages after each new release.