Satriani answers "For a guitarist giving lessons, they'll let a student through the door and say, "Can you teach him about modes?" When you're a teacher, you feel the pressure to get your thoughts together before turning that around and trying to get someone to understand it. That process helps you learn more not only about the subject but yourself and your own musicianship."
I found this an interesting comment and have heard it from several instructors. I also think that if you have the opportunity to give some sort of instruction it will help you become a better player.
The interview in it's entirety can be read @ http://www.austin360.com/music/content/music/stories/2008/111/1105satriani.html