Optional tools are things that you might not get any real value from. For example, you probably won't reap enormous benefits from using Flow to check for type safety or Storybook to isolate component development at the very beginning of a project.

The key thing to remember is that any React tool is optional, and no decisions are permanent. You can always bring in Flow later on, and you can always ditch Storybook if isolated component development isn't your thing.