Well, since TypeScript is a superset of JavaScript, all JavaScript is valid TypeScript. In other words, you're only gaining features by adding TypeScript. You can use as much or as little of it as you want.

That being said, most people use Typescript for -- you guessed it -- types. Developers coming from other languages such as C++ or Java where you must statically type all of your variables prefer Typescript as it lets them continue those development practices. In addition to types, it also gives JavaScript some missing features such as Interfaces and Namespaces. And actually I think now TypeScript also supports JSX.

Obviously it adds another step to your build process, but if you're writing ES6/Next code you're transpiling anyway.

Can I answer your question by asking a question: Should a developer switch from C# to Java?

Because vanilla JS won't even detect a typo before you run it. TypeScript gives you amazing tooling and huge productivity gains, making your code more reliable and dependable. You'll spend more time implementing features instead of tracking down typo bugs and reading the source of a function just to remind yourself of it's signature. That's why a vanilla JS should use TypeScript. As a JS dev turned TypeScript fan, I reject the idea that TypeScript is turning JS into Java. I think it turns JS into the beautiful language it was meant to be by taming it where necessary without sacrificing any of the goodness of dynamic typing when you need it.

As said by @nehero, you only have things to win, the module, typing, more readability of the code, type checks, and others. you can read if you want this article