Anything that uses JSX syntax, for instance.

Any kind of downleveling, though that's less important these days most users only need polyfills, new syntax features like `using` are not widely used.

Minification, and bundling for web is still somewhat necessary. ESM is still tricky to use without assistance.

None of these are necessary. But if you use any of them you've already committed to having a build step, so adding in a typescript-erasure step isn't much extra work.