Type system allows to mix type annotations into JavaScript code. This improves code readability, improves refactoring capabilities and code completion in IDEs, helps avoid runtime bugs.

TypeScript vs. Flow

Short comparison: https://levelup.gitconnected.com/flow-vs-typescript-in-react-my-two-cents-d4d0c657d236
More details: http://djcordhose.github.io/flow-vs-typescript/elm-flow-typescript.html#/