None, actually. If you know the material, feel free to skip my lectures.

The material I teach is generally considered "hard". Most students need my help to point out "This is the main path. Learn this. Let's go over this multiple times in different ways until you see the commonality. That stuff is auxiliary--ignore that for now. Once you have the main idea down, that stuff is just a little bit of extra work."

Every class is different. One class will get an idea very quickly while another will struggle--so my job is to tweak things for that in lectures. In addition, sometimes a class will have a huge gap that you will have to back up and fill.

Even with STEM subjects, teaching has a lot of art to it.

I get the impression that you and ska are very, very good teachers. Not to disrespect any of my professors, but a lot of times I ended up skipping lectures because I just wasn't getting anything out of them. Was I a model student who always reviewed material before lectures? No. But I definitely felt I got a lot more out of going directly to the source materials and working problems than I did watching a professor walk other students through problems.

It's unlikely that you fully understand the material from a single read.

However, if you can convince yourself that you do then by all means skip that lecture. You'll find out how accurate you were on the exam.

Because reviewing and being familiar with material is different from knowing it, and even if you know a topic, surely you can learn more.