This is a great question. Let me make a few point of how psychology of religion can help the teacher of religion.
First, we need to realize that most of the people in the world is religious. This might be an unbelievable statement, but it is true. People thought that religious would decline, but we are seeing an explosion of it. Hence, it would be great to ask the question, "why." An angle of that psychology provides might also be very valuable.
Second, the way our minds work when it comes to religion can be a very fruitful topic of exploration. This alone would merit teacher of religion to know something about psychology.
Finally, when it comes to supernatural things, there is a fine line between sanity and insanity. This intersection is worth exploration as well.