We need to move stuff around while making or updating courses. We can move around the units so the need is apparent. We also need to be able to shift around the pages inside the course. When I was an instructor at Learnable.com with my jQuery course this was very helpful. In some ways to me it is critical for producivity. Without it we spend too much time planning. When plans need to shift it is a painful copy and paste scenario, which means more moving parts. If we can sort them the moving parts don't get as many failure opportunities.