I actively remind my students this when I teach, who honestly are less active post pandemic. It is something I worry about. And though my account is "anonymous", if you ever show up to my lectures, not as a student but just sitting in (I don't take attendance, idk if you walk off the street and want to sit in), you too are welcome to participate.
(Fwiw, there aren't really "PhD level lectures" so much as graduate lectures. Generally you stop taking classes at the end of year 2 in your PhD, which is equivalent to a masters. The distinction of the PhD is that you are learning to learn on your own. This does add extra context to why grad students ask for higher pay, especially at the non-conditional and candidate levels, as they're actually taking less resources from the school. E.g. I pay the school $10k/term (big round) for research credits, which requires no involvement from anyone. There no longer is the "student" part to my "51% student, 49% employee" split)
Good on you for allowing auditing. I had only one teacher in college who allowed people to audit my classes and he even fought the administration to give me credit for the class after I sheepishly approached him and asked if he would mind letting me taking the exam to make sure I understood the material. One of the most insightful classes I took my entire time in college
A well designed class session can be (can be) similar in that it provides a space and guidance for you to do 60 minutes of improving your brain, that let's be honest, you would otherwise half-ass at home. (Of course, the classroom is only part of what you're paying for, there are also assignments, feedback, etc.)
(Now, I grant you, in a room of 300, interaction is severely limited. I'm lucky my classes tend to have fewer than 20 people.)
If I give a lecture that you could have just watched a video of, I fell short.
It's tough to get right. I'm still working on it.
Absolutely. Even in the largest lectures, people can ask clarifying questions, live, from the subject matter expert. Likewise, the professor can look at the students' body language and realize that they've lost a significant portion of the class, and stop, slow down, and maybe prompt the class for questions.
There is an art to a good lecture, and it's definitely harder the larger the class, but good lectures cannot be replaced by recordings.
1. Real life lecturer that can be asked questions
2. Peers that you can meet and discuss both study-related and unrelated topics.
3. Language. Not relevant for intros, but advanced topics can be explained in more than one way, and as a student you not necessarily will be able to map between them.
4. Schedule. Self-pacing 6 different classes for 12 weeks may be harder than it looks. Instead, you can just attend classes.
5. Advanced topics. Online resources, especially videos become scarcer as you advance in your studies, more so if you are not in hot topic (e.g., CS). I still have some notes from classes I took as undergrad as a best source for the learned topics. Good luck finding video lectures about diagrammatic approach to real time dynamics of open spin systems.
Same goes the other way around, sometimes attending class is just not worth it, even without tons of online courses. I learned my E&M by just reading the official book during the lecture time (see 4.).