In order to design a good presentation, you need to understand things one level deeper than what you are actually going to share in your slides. If you are the inventor of the product, or the university teacher of the subject, that is no issue. When you are a junior executive, management consultant or a professional presentation designer, it is not obvious.
The most obvious issue of knowing your slides but nothing more are questions. They will be difficult to answer. In management consulting firms, knowledge documents are shared internally and used freely across the entire firm. Modified versions of slides end up in all kinds of decks and are often presented outside of their original context. A few questions from the audience can undermine the credibility of the entire deck.
As a presentation designer, going one level deeper allows you to think about the story you want to tell from scratch. Often the expert has left out bits of the story that are obvious to her, but crucial for a cold audience to understand the full picture. Or, the expert goes on and on about facts and details which are not important. If your knowledge is not broader than the 20 minutes of content covered in your deck, you cannot choose what to include or not.
Clients are often surprised that I insist on trying the product demo, understanding the mechanism of action of a new drug, or dig into the financial model line by line. It is also the reason that I think presentation designers cannot charge by the slide, since there is a big fixed cost component to understanding a new story, independent of the slide count of your presentation.
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