For me there was a huge difference between a professor going through the process of deriving a formula on a board versus just having it flash up on a Powerpoint slide.

You could probably use Powerpoint to simulate that, but most professors don't. Even if you do show the intermediate steps, I think the gradual process of watching something being written out (as well as then having the time to write it out yourself) let's it sink in better.

Even something like having to decipher the professor's handwriting contributes to better understanding. If it's ambiguous what a particular symbol is, you're forced to think harder about the context it's in to figure it out. I'm not saying lectures should be given with obfuscated notes or anything, but that split second of interpretation your brain has to do adds value in the end.