any type as output of JS module. When I have more time, I could just submit PR to DefinitelyTyped and it would be quickly accepted. Overall it gives less ambiguity for my code.
Refactoring is a big one for me. You get almost instant feedback whenever you make a substantial change to the organization or implementation of a part of your codebase, even before you run unit tests and before you run the app.
Also it helps to have self documenting types that help you immediately see what kind of data some method expects and returns.