"My daddy's bigger than your daddy!" So it goes. It's always nice to have someone to back you up. Not only does citing an authority help establish the validity of your arguments, it shows that you're not alone, that you're not a singular crackpot who dreamed up this idea over breakfast. It also reveals your reasonableness: you took the time and effort to discover what other people have to say on this matter and you're willing to share your discoveries with your reader. You're a member of the Community of Scholars.
Citing an authority with an established reputation is better, of course, than citing someone whose credentials are not so lofty. Perhaps we assume too quickly that the opinion of someone with degrees after her name and someone who works for a prestigious university should have more weight that the opinion of good old Uncle Ronnie, but the writer should learn to take advantage of that assumption. Our reliance on authority needs a more substantial hold on reality and credibility than the world of advertising, where retired baseball stars tout mortgage lenders and golfers assure us of a tire's reliability and safety. On the other hand, beware! The Professor of Nutrition at a large midwestern university may take a position on farming practices because he's desperately trying to get a research grant from an agribusiness mega-farmer.
Be especially careful of authorities you cite from the internet (with the exception of this one). The library at the University of California, Los Angeles, has put together a document on the evaluation of resources on the World Wide Web. It might be wise to review that document before you rely heavily on internet research for any college papers that you write. There is also a site put together by librarians called Teaching Critical Evaluation Skills for World Wide Web Resources from Widener University. A tutorial document on evaluating resources of all kinds comes from Cornell University. Hope N. Tillman, Director of Libraries at Babson College, has published an extensive online essay on this subject: "Evaluating Quality on the Web," which contains several useful examples of evaluative choices. If you ever have any doubts about the reliability of your resources, consult a librarian.
Citing an authority also requires us to learn the habits of proper documentation. Study the Guide to Writing Research Papers for help. Learn how to use that wonderful phrase "according to":
"According to Mary Darling, Associate Professor of Nutrition at the University of Minnesota, . . . ."
Or, "As Professor Darling points out in the Journal of Native American Nutrition. . . ."
The other sub-sections of this part of Principles of Composition are as follows: