I find another benefit of blackboard presentation is the order of information. The lecturer can build something up rather than simply showing the finished article and saying voila.
I'm sure most people have seen professors skip numerous steps on blackboards, too, but in my experience there's at least a heightened awareness of what it looks like to actually work through the problem.
The point of going to classes versus studying alone at home is that you get to watch mathematicians doing mathematics. It's apprenticeship learning, the same as carpentry. Watching the professor reason and make mistakes and second-guess himself is invaluable. The typed class notes where everything is neat and theorems are numbered gives the impression of an immaculate birth that the student can't fathom arriving at. The best maths classes are the ones that teach you how to be a mathematician.
Everyone jokes about the frustration of copying a full blackboard worth of notes and then having the professor go "wait no, that wasn't right" (myself included) but it's a valuable experience. That moment of "hm, hold on a second" is where the math is happening, and a chalkboard helps professors and students notice that. (And of course, when something "isn't quite right" on the Powerpoint, it's a tremendous pain to fix...)
A great teacher will be a great teacher with whatever you give them. But anyone not 100% tuned to that part of their teaching greatly benefits from some of the constraints that chalk gives them.
I'm a K-12 admin and once we had a HS teacher who was brilliant but her classes were often too difficult to follow for the kids. We just told her she couldn't use the powerpoint slides anymore. Yes, there was a huge discussion of timing and pedagogy and all of that, but ultimately, placing that constraint on her naturally put her at a much more equal footing to the students she was trying to teach.
Content aside, it was a very interestingly run course. Assignments were at your own pace, you had a large list of the kinds of projects you could turn in for credit. There were quizzes, exams, essay topics, interview topics, and an option for independent projects to get credit. Each kind of project had it's own maximum point value it would be graded out of and the final grade was totally based on how many points your actually earned, not out of how many. Even if you did a shit job on every assignment, as long as you did enough of them, you could get an A. It was a nice way to boost the GPA from just sheer effort rather than actual achievement.
The people who don't show up would have not showed up class recorded or not. It is very useful though when you have a morning class miss or be dead tired and not pay attention to re-watch the lecture to get a better understanding.