Certainly the business case is important to lay out, especially in the case you describe. Many decks don't do that though - they don't stand alone without a verbal pitch. They lack context and sometimes even topic headings.

I've seen many decks that have a page with a heading like "Opportunity" and a number (along with the obligatory graph going up and to the right). What is that? Competitor sales? Expected market share? Entire market size? Even if the investor is watching you pitch out loud, they may refer back to the deck and find they have forgotten what you said about that slide.