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, or web site.
The keys to successfully citing your sources in APA Style are:
This guide, prepared by the Business Librarian at Brock University, includes detailed examples of references from some of Brock University Library's Business databases, as well as guidance on citing sources and images in presentations.