I always thought taking notes was a waste of time. Surely you could just look back at the slides if you forgot something?
But I realized I might have had it backwards all this time. What if you take notes not to have something to refer to, but because it forces you to listen with greater attention and strengthens your memory?
To really take notes you need to understand the subject well enough to pick out the key phrases and concepts in real time, all while the lecturer is speaking on and on. It's similar to how actually doing the exercises in your textbook will make you realize very quickly which parts you've understood, and which parts just sounded vaguely plausible.