Borrowing a memorable line without attribution is plagiarism. By including the source, we show integrity and professionalism in research and presentation. When expert opinion is helpful. We can lend power to the information by disclosing its origin and the authority behind it. When it sounds too good to be true.
Illustrations sometimes sound contrived and, therefore, manipulative.
In the case of an amazing but true story, citing the source makes it more believable. When local people are involved. Given opportunity to quote a hometown hero, the inclusion of the person's name will stimulate interest in the rest of the presentation. When the sermon is distributed in print. In such cases, full attribution is a must.