edit: obviously talks that include slideshows, usually on the back of the presenter.
Then there are progressively worse scenarios, like just streaming from a camera that was placed somewhere in front of the stage, recording a specific viewport. Or the speaker full screen and the slides in a tiny box in a corner.
If these things are separate in the stream/file, then the client software would be able to have options/layouts for what to display where, resize/move things, even split out in multiple windows (for multiple monitors).
Basically, a multi-layered video/presentation format.
PDF would make a nice layer to that, being able to change slides based on the timeline of the feed... or even change the slide manually, and it navigates the timeline to that point.