Without the '--keep-video' option the two files would be downloaded again and even using the option, ffmpeg would be run again, which for some videos can take a long time.
We use a temporary file with ffmpeg so that the final file only exists if it success
line 116, modify 'Downloding' to 'Downloading'.
It looks like nothing, but it is very annoying when youtube-dl command's output is parsed to find progress on a php (or other language) website for example.