It is important to conduct an audience analysis prior to giving a presentation so that your listeners will respond as you wish. The audience knowledge is what level they are at and how much information you need to give to them. What do they want to know? What do they need to know? It's important to satisfy their expectations. Otherwise, my presentation will be seen as a failure for them.
If my purpose was to persuade my audience to do something, their behavior after my speech can tell me if I did in fact persuade them.