Example for visual aids
This simple graphic from Hot Butter Studio is an excellent example of an effective use of visual aids.
I emphasize to students that visual aids should not just repeat what the speaker is saying (although their professors will often use them this way to simplify note-taking). However, most audiences for speeches are not taking notes because they expect to be tested over content, so visual aids should only be used when they can ADD something that the words alone cannot provide.
One effective use of visual aids is to help the audience visualize numbers. Simply listing the numbers will not be as effective as putting those numbers in a meaningful context, as this example illustrates.
In addition to showing the example in class for how and why to show data, it could also be part of a class activity where students analyze a series of visual examples for effectiveness — what does it get right, get wrong, or whatever your textbook gives as standards for good visual aids.