In most openings, speakers are trying to accomplish two goals.
The first goal is to state what is the main purpose of the speech. Depending on the length of your presentation, you may have several main points to this purpose. In your opening, a very concise explanation of your purpose is needed. As an extra benefit, this explanation of your purpose allows you to give a blueprint of your speech to your audience. This allows your audience to know where they are starting and where they will end.
When you explain your main purpose, this leads into your second goal. Your audience has to know that there is something of value in your speech in order for them to listen. No value means no need to pay attention. Every audience wants to know that when they leave they have gained something of value. Every speech has something of value, it is our job as speakers to let our audience know what it is.