In advertising, most companies use testimonials. That's when a person (sometimes a celebrity) endorses a product. When companies give out pamphlets about their product, sometimes the brochure will have comments by people who are bragging about the product and giving reasons to purchase it.
Examples:
" The children are enjoying their new math drills. ... [the students] are enjoying the challenge of beating the computer to the correct answer ... "" John Smith, Los Angles, California
That's a testimonial. "John Smith" is telling everybody that Math Drills is a great software, giving everyone a reason why you should buy Math Drills. So then they know that you'll have proof of the fact that Math Drills is a good software.
You may have already noticed that they don't just pull people off the
streets to give testimonials. The big companies use big celebrities
like Rosie O'Donnell or Tim Allen to advertise. For more info, see
celebrities.