Personally, for me it's archiving. In case I want to go back to it. Videos just keep disappearing from YouTube because channels get deleted by YouTube, by their owners, videos get copystriked, geoblocked, privated, and so on. As I'm lazy as f### I didn't create anything as sophisticated as OP, but a simple 10 line PHP Script on my home server that just pretends being Kodi enough to fool yatse (android remote for Kodi). So every time I watch a video on YouTube (on my phone) that I want to keep I tap "share" and then "play on Kodi", my php script gets the video url from the post data and launches youtube-dl. It sucks because I never get feedback if it worked and when it's finished, but I log all the URLs and at some point in the future I'll eventually add a cronjob that checks the list and sends reports and whatnot. Some day.