The importance of tests was already described in the Project Setup chapter. If you want to make your workflow effective and consistent, you should definitely start writing tests.
It’s a very useful investment which, besides guarding your code quality and helping you ship new versions without bugs, also helps you during the refactoring.
It’s crucial to have tests, especially when you are coming back to the project after some time. It happens that you just don’t remember some things. And tests could serve as documentation.
Usually, the absence of meaningful tests makes it almost impossible to take over somebody else’s codebase. If you do so, you will have bad velocity on a project. And you won’t be sure if you’re risking breaking something by making a change in the code.