In public speaking keeping note cards is imperative to have to keep on track and minimize tangents especially if you have a large amount of information on the subject. I have found that in teaching and presenting that the same is true. A prepared set of material to be covered in each days lecture helps an instructor to give the perception of being prepared and ready for the task at hand. Also a good outline of the information to be covered assures that material should not be missed as well. On the other hand A presentation can be ruined if the presenter is using the note cards as a shield and reading from them entirely. In my experience I have also found that the preparation of the notes allows me to finalize the presentation in my memory and allow me to get back on track if an interruption occurs or there is a pressing question that needs to be answered.