According the Arizona State University LibGuide on Citation and Plagiarism, there are four main reasons to cite:
The main reason not to plagiarize is because doing so is unfairly attributing ideas of someone else to yourself, whether or not you intend to.
If you’re unsure whether or not to cite something, ask your professor or a librarian for help. Remember, it’s better to overcite than undercite!
From the University of Tennessee - Chattanooga
What is plagiarism?
Plagiarism is using someone else's work without giving him or her credit, leading your readers to think those words are yours. While this might seem easy to avoid, many people who plagiarize do so unintentionally. Although most people think of plagiarism as recording someone's exact words without crediting him or her, it also includes re-arranging someone else's words (paraphrasing) or using his or her ideas. These forms of plagiarism are far more common and require careful attention to avoid.