Have you noticed how, whenever you start reading an article with a promise in the headline "The 18 secrets to [x]", "Why is it that [y]", you usually skip the introduction and/or skim the text to find the answer the headline promised? Introductions usually repeat the headline and contain background information such as the bio of the speaker that we do not have time for. Not the interesting stuff the reader is looking for.
Your audience wants to do the same with your presentation, except, they cannot. Taking the clicker, fast forwarding and asking you to get to the point would be rude. Instead, they start checking email on their mobile device until it gets really interesting.
Detail of an image claimed by John McNab.