I do it because I might want to watch a video full screen in one monitor and continue to work on another. Embed automatically takes up the full window. An added side effect is that the pre-roll ads disappear. The banner ads are still there.
I just automated the downloading and playing of Youtube videos from the command line. Here's the script that'll do it: https://gist.github.com/lelandbatey/7594106
I do actually have that installed (or at least something like it) I actually use it a fair bit for things I want to watch but don't want to listen too. Somewhere between comcast and youtube I find it nearly impossible to stream 1080p quality videos. I feel like youtube doesn't buffer like it used to.