It's an aspect of self that keeps you from venturing out farther than you're psychologically prepared to handle. For example, I was shown the entrance to a hellrealm last year. I was accompanied by a guardian, and was able to see what I was supposed to see and learn what I was supposed to learn and not freak out. If I had "accidentally" stumbled there three years ago all by myself, I'd probably be in therapy now. The watcher keeps you safe from your own curiosity, and also helps to teach you lessons. But remember, it is an aspect of self. I'm sure AC has more info if you need it.