> It doesn't even ensure type safety because there is no runtime type validation for JSON objects received from the API.

It ensures internal type safety. If you don't have a lot of complexity on your side (e.g., a large client-side app) that may not be worthwhile to you, but it's certainly a thing it does, and a reason static typing is generally considered to be of value, particularly in large, multiprogrammer and, even moreso, multi-team projects.