A combination of 2 reasons:
- I prefer to watch “entertainment” videos on my TV set – movie trailers, music videos, etc. – using my Apple TV. I find the YouTube app for Apple TV to be abysmal – not just for searching, but also for bookmarking. So instead, I download the videos to my Mac, and watch them from my Apple TV using home sharing. A much more pleasant experience.
- I also like to curate a small collection (~100 videos) of said “entertainment” videos for repeated viewing. So I store my faves on the Mac, and when I’m streaming TV shows or movies on my Apple TV, I may take a short intermission between episodes, and watch a video out of my collection. (A throwback to the days when they showed cartoons and shorts before movies in theaters.)
For informational YouTube videos, I’ll watch them from Youtube via the web browser on my Mac – which is the majority of the time. But entertainment videos are a corner case for me.