I had the same question initially. TypeScript is the way you configure intellisense. Some types can be inferred, but not all, and most of the typing I do is to _restrict_ what data can be used. TS gives you feedback anywhere you ask for it, and paired with IntelliJ I feel like I have superpowers.

Another point: You can create types for your API client programatically, meaning your front-end can be aware of exactly what types your backend is returning. This cannot be accomplished with intellisense.