Appeal to authority. From childhood, children have been trained not to dispute their elders, the authorities in their lives, and that training continues to condition adults to do what authorities tell them to do without thinking about it. No more than what "My Dad said..." or "My teacher said..." could be wrong, can anyone else who is presented as an authority.
Like parents, employers are considered authorities because of their power over employees; government, because of its power over the people. In personal disputes, one party will assault the other's opinions by claiming to be or quoting someone claimed to be an authority.
In truth, however, there are remarkably few real authorities. A real authority in any field is a truthful, conscientious expert. Even that is no guarantee they're right. In criminal trials, it is not uncommon for both the defense and prosecution to present expert witnesses who give completely opposite opinions.
In our culture, doctors are considered to be authority figures, so much so that few patients question their doctor's professional reputation, qualifications or credentials. Neither do they ask for a second opinion, question a diagnosis or dispute any advice regardless of how ridiculous it is. In TV commercials viewers hear "More doctors prescribe..., More hospitals use..., More dentists advise...."
Or just an actor wearing a white lab coat will imply an endorsement by doctors and the implication is enough. Endorsements by Hollywood stars, sports figures, etc. imply both that they are authorities and that their saying so makes "this" the product to buy. Direct mail promotions rely heavily on endorsements by those who claim to be authorities.
Use of testimonials stems from another of the fallacies.
In advertising and sales, fallacies are often BASIC, FUNDAMENTAL RULES!