I agree with you in some respects, especially if you have small private methods, typing everything may have diminishing returns.
What sold me on typescript is that it can be used without typing and automatically start giving intellisense and compile errors when passing invalid parameters. So you can add value, even without typing. Then, some models are reused throughout an app, typing commonly utilized models makes things easier for developers to understand the public methods of your well designed, encapsulated classes, without ever having to look at the code to understand the data needing to be sent in. This can be true with primitive types also, on public methods used throughout your app. Having a method clearly indicated as accepting a certain type of data not only gives you type safety which can prevent bugs, but it also make it easier for other developers to understand the code without opening up the class and reading it.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.