I have shitty internet and there are many times of the day when its hard for me to watch videos without them always buffering. youtube-dl lets me download the videos on the background so I can watch them later at a decent resolution and without all the buffering.

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).