Answer:  Consider the possibilities:

a. You have a photographic memory and have total recall of anything you see; then, there is no need to take notes.

b. What I do in class is done to dazzle and impress you and is not designed in any way to contribute to your understanding of the material; then, sit back, relax, be dazzled and impressed, but don't bother to take notes.

c. The premises of 'a' and 'b' are false; then, take careful, detailed notes that allow you to reconstruct and study what you have been shown in class.