Citing informs your readers, including your professors, of the sources for your information, ideas, and thoughts. At the same time, it credits the author for the original information. Citing respected sources confirms that you have thoroughly researched your topic, and it also strengthens your arguments.
Equally important, failing to cite properly can result in plagiarism - using another person's work or ideas without crediting the person. Plagiarism is a serious violation of academic and professional ethics, as the box in the right-hand column explains.