Why use ffmpeg to convert mp4 before youtube upload

[patcon]

Zoom2Youtube/src/youtube.py

Line 88 in 91fc12a

'ffmpeg', '-i', fpath, '-crf', '18', '-preset', 'veryfast', '-c:a', 'copy', out_fpath

Hi! I'm unclear why this happens. Any quick explanation? Not seeing much context in the commit logs :)