How Does Plagiarism Affect the Integrity of Knowledge?
When people plagiarize, the original source of the information or idea is obscured. This is true even if you do not copy from someone else but just include information without indicating the source. If we lack a clear understanding of the sources of knowledge, the knowledge itself is not reliable. Misinformation not only wastes a reader's time, it calls into question everything else in a piece of research and any future research built upon it.
Imagine you are writing a paper on the history of tobacco farming in the Pioneer Valley. It will be important to indicate whether the tobacco farmer who wrote one of your primary sources lived in the Valley or in Virginia. Your readers will also want to know if a secondary source you use was written by a professional historian, an agricultural scientist, or an accountant who is also the granddaughter of a Pioneer Valley tobacco farmer. Each of these people may have something helpful to say about the subject, but you would be better off relying on the historian in discussing social change, the scientist for questions of soil quality, and the granddaughter for personal details about her grandfather. Your readers should also care whether the book was published at Kinko's or at Cornell University Press. If you take information from the Web, it matters whether you use a hobbyist's personal Website that lacks citations or a carefully researched Website created by a historical society. How will your reader be able to evaluate your conclusions if you do not provide all of this information in your citations?