It's used to prefix the stdout / stderr output of galaxy-api and persona-faker and then forward that output to stdout / stderr for this process. It's used [here](https://github.com/cvan/galaxy-api/pull/88/files#diff-bc734efbd73721c31fdf3f59821df192R135). I originally just created a `stream.Writable` and overwrote `Writable._write` directly, but it was pointed out that that was a sketchy thing to do.