A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Reasons for citing your sources
General introduction to citation practices
Part II. Source Citation

The first duty of a researcher is to get the facts right, but a second duty is to tell readers where the facts came from. To that end, researchers must cite the sources of the facts, ideas, or words that they use in their papers.

15.1 Reasons for citing your sources

You cite sources for at least four reasons:

1. To give credit. Research is hard work. Some who do it well receive concrete rewards—money, promotions, good grades, degrees, and so on. But no less important is recognition, the pride and prestige of seeing one's name associated with knowledge that others value and use. In fact, for some researchers, that is the only reward. So when you cite the work of another, you give that writer the recognition he or she has earned. (You also guard against a charge of plagiarism; see 7.9.)

2. To assure readers about the accuracy of your facts. Researchers cite sources to be fair to other researchers, but also to earn their readers' trust. It is not enough to get the facts right. You must also tell readers the source of the facts so that they can judge their reliability, even check them if they wish. Readers do not trust a source they do not know and cannot find. If they do not trust your sources, they will not trust your facts; and if they do not trust your facts, they will not trust your argument. You establish the first link in that chain of trust by citing your sources fully, accurately, and appropriately.

3. To show readers the research tradition that informs your work. Researchers cite sources whose data they use, but they also cite work that they extend, support, contradict, or correct. These citations help readers not only understand your specific project but connect it to other research in your field.

4. To help readers follow or extend your research. Many readers use sources cited in a research paper not to check its reliability but to pursue their own work. So your citations help others not only to follow your footsteps, but to strike out in new directions.