For example, Steve Jobs keynotes made you go to the Apple Online Store and preorder the latest products; Bret Victor in his "Inventing on Principle" talk makes you rant about the current state of IDEs.
The effect of a talk disappears rapidly after the speaker has left the stage. In contrast, a written text stays.
I couldn't have put it more succinctly myself.
Motivation is speech's primary function. Getting you to vote, or buy something, or work harder, or learn something. Everyone who is trying to get a group of people to do something is using speeches at some point.
The effect of a talk does not have to "disappear rapidly" after the talk is over. That is why you will find lots of old Jobs keynotes on youtube, because they are still very powerful, and useful, interesting, educational, and motivating even when the products discussed are no longer being sold.