This is a great add-on for saving videos on your own system to play when you want, without having to search for (and maybe not find) the one you're looking for, and then stream it, and put up with the adverts that interrupt it!
I have been using it for years, and it is continually updated (praise to the author for that), primarily due to some of Youtube's code changes, apparently designed to suppress the ability of add-ons like this to download the videos.
If it stops working, it's not because it's broken, more likely because of Youtube code changes - just get the latest update to get it working again.