I know one reason for it. Speed. I used to watch videos of professors where they were taped giving normal lectures. One learns a lot that way if all you are interested in is that one course and need no credits for it. I was particularly enthralled by the Berkeley videos and one guy gave an excellent course on physics.
I tried another course where a professor was giving a lecture on basic anatomy. Now that subject can be very boring to begin with but this woman stood there and hand wrote practically everything she was saying. I could have bought a book on anatomy and read it faster than she was writing on the chalkboard. Even when she pointed to the skeleton, the camera didn't zoom in so you couldn't tell what exactly she was pointing to. Worst teacher I ever encountered. She didn't even draw well.