On this day in 1963, Martin Luther King, Jr. gave his "I Have a Dream" sermon [often called a "speech"]. He had stayed up until 3 a.m. writing it, trying to get it just right. It did not, in its written form, contain the sentence "I have a dream".
If listening to a recording of the sermon, the listener will hear King pause. From the moment he speaks those famous words, all that follows is delivered extempore as he surrendered to the Holy Spirit. Hence, I suspect, its power.