Whenever you quote or base your ideas on another person's work, you must document the source you used. Even when you do not quote directly from another work, if reading that source contributed to the ideas presented in your paper, you must give the authors proper credit.
Citations allow readers to locate and further explore the sources you consulted, show the depth and scope of your research, and give credit to authors for their ideas. Citations provide evidence for your arguments and add credibility to your work by demonstrating that you have sought out and considered a variety of resources. In written academic work, citing sources is standard practice and shows that you are responding to this person and adding something of your own. Think of documenting your sources as providing a trail for your reader to follow to see the research you performed and discover what led you to your own conclusions or contributions.