What do motivational speakers, salespeople, preachers, and lawyers have in common? They are all in the business of convincing people, and they do it well. In fact, they do it so well, that this is their job - to persuade people to "buy" what they are selling, whether it's an actual product, or a concept, a story, an ideology, etc.
A motivational speaker's job is to convince you to such an extent, that you become motivated and determined to follow the steps that they are explaining to you. Persuasion is their "bread and butter".
Salespeople are the same; their livelihood depends on persuading you to buy the products they are advertising. They are skilled at their craft and employ various psychological techniques to convince you to cave in and make a purchase. Think about car salesmen and how well-known they are for their selling abilities. You have to be pretty good to persuade someone to buy a used car. Pretty impressive, right?
The stakes are much higher when it comes to people like lawyers and preachers. Their job is to convince judges and juries and entire congregations, respectively. Their oral skills have to be impeccable, as well as their delivery, a tone of voice, facial expressions, and penetrating gaze. Their stance admits no hesitation; they have to inspire unmovable determination and confidence. That is ultimately what sways people in their direction.