Honest question: why would you want to download a video from YouTube? The main use case I can think of is you want to watch videos offline on a mobile device but don't want to pay for a YouTube Red subscription.
I don't do it very often (except for mobile watching without burning quite as much battery) but I see a number of use cases.
I frequently backup entire channels using youtube-dl just to avoid the chance of that content getting destroyed.
I actually do pay for Youtube Red, but I like to download my Youtube videos because a) my internet connection is a bit spotty, and b) I like to keep as much as I can open source.
youtube-dl is also helpful for other video serivces. For example, twitch.tv doesn't have as good of a CDN on my country as youtube does so youtube-dl is sometimes the only way for me to watch a twitch.tv VOD at decent quality (even in those days when I can watch youtube videos just fine).
Honestly, those are the only scenarios I've seen in the wild. It's not like we are still living in the days of Napster.
Sometimes one may be in area of good connectivity, but often not.
This is commonplace in countries with limited internet access.