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.