How do convincing speakers vary their speech when they speak for different purposes and to different people?

Convincing people is very important, especially while you are doing a speech. A lot of people don't notice this but convincing is actually one of the main factor of a speech. Let's say you are presenting your product to a crowd, even if the product you are selling is very good top quality, most people won't buy it unless you make a good impression by trying to convince them. But it is not as simple as it seems because everybody is different therefore you are going to need more than only one way to convince your clients.