Because often the instructor or professor may ad lib based on a question from a student. It may result in a really good answer that probably won't be in the off

Because often the instructor or professor may ad lib based on a question from a student. It may result in a really good answer that probably won't be in the official lecture notes. Instructors with decades of experience have anecdotes that may not be suitable for lecture notes but may give express an idea better using analogy that is spur of the moment. I went back to college late in life and now with cellphones and cloud storage people often recorded lectures with permission. One person used OBS most others used their mobile phone's note recorder app. Many used Microsoft OneNote (free Office 365 with tuition) to record and photos of whiteboard notes and diagrams. OneNote can OCR the text and even adjust skew of the image. I still wrote my own notes though at first hand written because I wrote in cursive which is far faster than block printing. But I was persuaded by the masses typed my notes into a document on my laptop. I regret that now because I noticed a significant difference in retention between hand-written versus typed. When I wrote by hand I recalled much more of the material. And take this with a grain of salt because I'm friggin' old.