Primarily, it is to avoid allegations of plagiarism and academic misconduct.
Secondarily, citing sources allows the reader to identify the sources that you relied on for your research.
It's easy: whenever you paraphrase the ideas of others, whenever you use a direct quotation, or whenever you cite a fact that isn't common knowledge, you need to cite your original sources, whether they are books, articles, lecture slides, material posted in Sakai, personal communications, or a web site.
The keys to successfully citing your sources in APA Style are:
Brock University's Academic Integrity Policy provides clear expectations for the ethical use of sources and examples of dishonest behaviors including "Using direct quotations or large sections of paraphrased material without acknowledgment" (See Appendix Two of the policy for a complete list).
Some students might be confused about when to quote, when to paraphrase and how to avoid plagiarism. The following sources may provide additional insight into the difference between summarizing, paraphrasing, quoting and plagiarism, so that you can give appropriate credit for the sources that you rely on in your academic work.