ffmpeg "-pix_fmt yuv420p" option when streaming on Youtube

The official tutorial https://trac.ffmpeg.org/wiki/StreamingGuide uses the -pix_fmt yuv420p option. I used it too, copied it from the tutorial, and streaming from FFmpeg to Youtube works for me. However, it is not clear what it is for, and especially if it is a necessary option with Youtube, or if there is a better one. The Youtube documentation on the subject doesn't seem to cover this, or maybe it does but I haven't understood where: https://support.google.com/youtube/answer/1722171?hl=en . Does anyone know of any accurate documentation on this?