Things are always at their best in the beginning.
Powerful speakers start powerfully. They engage their audiences immediately. You must gain the audience's attention and interest the moment you walk on the stage. Without that attention, you won't get your message across, you'll have trouble sustaining whatever interest there is, and you won't have established your leadership and control—the keys to being a powerful speaker.
As a speaker, you are an unknown quantity, but only for the first 30 seconds of your speech. After that, everything you say will be heard in the context of that first impression, those first sentences. So it's very important to memorize your opening and practice it many times. Because this is your first contact with your audience, you need to keep from looking down at your notes, which does not help you seem warm, powerful, or persuasive. Your eyes are the most important way to keep people's attention, so make sure you know your opening well.