Whenever you use direct quotes or your ideas are based on another person's work, you must acknowledge the original source. If you don't, you are committing plagiarism.
Acknowledge authors for their ideas and hard work, show the depth and scope of your own research, help others locate and explore the sources you consulted, and ensure that you avoid committing plagiarism.
Citations provide support for your arguments and demonstrate that you have sought out and considered a variety of resources.
Responding to and commenting on the work of scholars shows your participation in the research process. Adding ideas of your own highlights your critical thinking skills and understanding of the topic.