FWIW, Flow allows you to put the annotations in comments, but the syntax for the type annotations is generally more pleasant to work with.

I have found the types to be useful in a large (for JS) codebase with more than a handful of developers. The types help catch bugs, especially when refactoring, even with a decent test suite and linting. Plus, using React and GraphQL, we get static types all the way from data coming from the server through to the UI.