Writing a full speech is a process that excludes the audience, whereas delivering a speech with limited notes involves and incorporates the audience into the experience. This concept is critical, because humans are more apt to give attention to speakers who seem to, or actually do, demonstrate a sincere interest in them.
"Speaking spontaneously, with authentic conviction and awareness, signals that you have something to say — a point you feel so strongly about that you’re willing to express it personally and out loud."