"This is intended to provide a visual view of the sourcemap generated by the compile, so that REPL users can see exactly what transformations map to which generated output.
One immediate impression, having done this work so far, is that the sourcemap is fairly sparse for this purpose: it will correlate specific symbols in the source, but the helper chunks are unrepresented. Having those linkable to the source would be 💯 ."
See https://github.com/babel/website/pull/1655