As we saw previously, there are many cases where we can and should run tests for responsive visualizations. In any case where we can write automated tests, we prefer automated tests over manual tests. The reason we prefer automated tests is easy: to make them repeatable.
Once we have automated tests, we can run them every time a change is pushed to the repository. If they are fast enough, we can even run them every time we make changes to the local source files. This gives immediate feedback about the functionality of the code, and fast feedback. Debugging is easier, identifying and finding bugs, which automatically makes you more productive and development more effective.