A Conundrum

Sometimes, teachers have student teachers. When said teacher is absent, though, the school still calls in a substitute. I don't understand this. There is already someone there to watch the class--teach it even--yet someone else has to be called. Why am I called to supervise someone who is obviously more qualified than I am? I'm not complaining too much, because I like the money and all, but it doesn't make sense to me. Today, I substituted in a girl's gym, and there was a student teacher. I didn't have to do anything except try to stay awake. It was interesting to talk to her, and I did learn a few things, so that was a plus side to it. It just seems to make more sense to use the student teacher rather than calling someone else.