How do you refactor code, if you have a poor test coverage? Also for me actually the most importan benefit is the instant feedback I get, when I write the unit tests before the implemenation. I can move faster with more confidents.
Probably by working in an environment where that confidence is conveniently provided by static type analysis. If the parts are sufficiently reshaped to fit together again you just know that it will work. And chances are that feedback is an order or two of magnitude more instantaneous.
