Testing and Continuous Integration

I am new to React and was also learning testing as well as continuous integration and its importance. You know, I have read a lot of articles on the topic of testing and CI and majority of them say that "both are very important":) and but really want you to confirm if my understanding is correct. So, firstly, concerning testing, is it true that we write tests in software development to ensure that we keep our application under control, that is, if we add new feature to the app then we run tests to ensure whether we break something or not. Again, we make changes to the app then run tests to ensure whether we brake something or not? Secondly, concerning Continuous integration, is it true that CI is needed so that we do not manually run tests, that is, we do not need to type npm run tests or we can forget to run tests before pushing to github thus CI runs tests for us?