We don’t teach primarily facts in lectures. We have proofs, at least in pure maths lectures, and we need to teach our students what a mathematical argument is, by developing such an argument in front of them.
If you have ever been to a talk or lecture where a proof or argument is displayed all at once on a slide, you may see what I mean: I personally find a proof very hard to follow when it is presented all at once, and it makes it impossible to say things like “What should we do next?” “We’re actually trying to get to this, so this next step seems sensible” or similar. Obviously, if you want to show a lot of data, a picture/diagram, an animation, etc, blackboard or real-time writing on overheads is not the way forward. But for proofs, I think it is the best way to go, and I know a lot of mathematicians agree with me (though possibly not all, I haven’t asked all).
Should we be teaching new material in lectures? I have heard the argument that in the days of the printing press, lectures are really not needed any more for transfer of information from the lecturer to the students’ pieces of paper, but should be about understanding. Read more about this in “Should we give out lecture notes”. I am trying to compromise by telling students what will be in the next lecture (in my blog), so they can read ahead in one of the many good maths books that are around, so they don’t have to have it as new material if they don’t like that way.