[stiiv]

In my opinion, JavaScript's main advantage is that it can be run in browsers in addition to other (i.e. server) environments.

If you're building something simple, and the type/shape of function parameters is either very obvious or very well-guarded, TypeScript's returns diminish.

However:

As a reviewer of complex code, I want to see annotations so that I can better understand what I'm reviewing.

As an author of complex code, I want to clearly communicate the intent of functions I write, so that they are not misused by other developers.

As someone who supports the use of my software, I want to reduce errors by guaranteeing the correct types/shapes of function arguments.

...and I'm just getting started. These are all very compelling needs in my experience.