It's also strange in that I don't think I'd get very much out of simply watching anyone work, unless they were also giving live commentary or something.
There was an article posted on HN a while back that claimed that the best way for beginners to learn a new skill is to watch an expert just work, without any commentary or explanation. I tried finding a link, but couldn't come up with working search terms on DDG, Goog, or HN.
That's where pair programming can shine: when I see a junior who need help, I'll ask if they don't mind me coming around and helping. Just asking the right questions and guiding them to find a solution to their problem usually do the trick. And it helps me see the pain points of our code, documentation and specs.