[OscarDC]

Transpilation is here a necessary step to test the application because e.g. his browser won't be able to parse raw TypeScript code.

Typechecking is not: the browser doesn't care about it, it's mainly to help the developer verify its code.

So to speed-up the build during development (to have faster iterations) the idea is often to make the building process only about the build by removing "unnecessary" steps like type-checking from it, while having a separate linting / typechecking etc. process, which could even run in parallel - but not be necessary to be able to test the application.

This is often done by using tools like a bundler (e.g. esbuild) or a transpiler (babel, swc) to erase the types without checking them in your bundling process.