"Why would you want this?
Well for starters: on a small screen it gives you some extra height to the display. It also makes your desktop look really clean"
I couldn't agree more with Ben. I don't use my Mac much, but still got around to running the El Capitan beta over the summer, and have had my menubar off ever since I realized it was an option. It's worked well in my (somewhat limited) use.
I'd probably be as happy as Ben about this option's addition if I used my Mac with any frequency, but with my iPad-centric workflow it's just an option that's nice to know is there.