1. Give credit where credit is due
Every time you use someone else's ideas or words, you need to give them credit. Whether you are paraphrasing or quoting, no matter where you find the information (journal article, book, Wikipedia, website, etc.), you must cite your sources.
It is plagiarism if you use someone else's ideas or words without crediting them. Visit the Library's Avoiding Plagiarism guide to be sure you understand plagiarism.
2. Help your reader find your sources
By citing, you are providing your reader with the necessary information to locate your sources.
3. Lend credibility to your arguments
Citing sources demonstrates that your arguments are solid, and backed up by other research.