There are a number of good reasons a speaker should consider the nature of the audience to which one is speaking. Here, I'll limit myself to explaining just one reason.
On the most basic level, what one says and how one says it in a speech, lecture, or any other sort of presentation is determined by the presumed knowledge of the people in the audience.
As an example, a speaker who is a physics professor would give a very different presentation to an audience made up of members of the general public - in other words, people who are not involved with the subject of physics - than he / she would if the audience was made up of fellow physics professors.
In this respect, the question a speaker should ask him / her self is how much knowledge about the topic of one's presentation is the audience likely to have. To be a successful speaker, one has to make the subject of one's presentation intelligible to one's audience and, therefore, a speaker should anticipate what he / she will need to explain to the audience.
As a military historian, for example, I know that I don't have to explain the concepts of, for instance, tactics, strategy, or logistics when I'm giving a presentation to a group of fellow military historians. If, instead, I was giving a presentation that included these same concepts to a group of chemistry professors, I wouldn't assume that they would already be familiar with the concepts of tactics, strategy, or logistics and, therefore, I would have to include explanations of these concepts in order for my audience to be able to understand the contents of my presentation.
