Question

1. Why do advertisers use testimonials?

Answer 1

For the first one, the best answer is that they:

They provide celebrity encouragement to persuade the consumer.

they don't always contain lies, but also they don't need to contain the truth, they contain an opinion.
The second one:

"Use full body movements to improve your health."."

this is the only one that actually mentions a sports health - the other ones focus on the appearance rather than health