The most important reason to cite sources used for your assignments is to avoid plagiarism. Plagiarism is synonymous with stealing, and can result in a failing grade if caught. Some schools and instructors have even harsher penalties, such as a mark on your permanent record, suspension and even expulsion.
While failing to cite your sources, or using text and/or ideas that are not your own constitutes plagiarism and deserves applicable punishment, it is also personally unethical. Being a person of integrity with high morals is important in all areas of life -- schoolwork included. If you wouldn't steal from a store, do not steal from other sources such as a literature review or the Internet.
Citing your sources also gives your paper tremendous credibility. By providing outside information, readers of your work know you have done your research and are an expert of sorts on the topic. Gathering the opinions of experts and finding statistics and studies to support your work adds authority, and makes readers trust your work and conclusions.
Citing is important because you need to know how to cite things properly to achieve the highest possible grades. Instructors can take off many points for incorrect citing because the resources are easily accessible for you to utilize. Citing improperly boils down to laziness. Use websites like the Online Writing Lab provided by Purdue University to verify you format each citation and your entire bibliography correctly.