> I don’t get the idea of saying one thing while the audience reads something else. It seems like a sure recipe for confusion.

If one does this, it should be because the slide is a list of short bullet-point talking-points, and the speaker is going through them one by one explaining them in more detail.

> I usually put almost no prose-type text on my presentations. Figures, captions for figures, equations, variable definitions, small code snippets, those are ok.

This is honestly the type of stuff I always found most helpful in a slide-deck presentation. Information that enhances what the speaker is saying.