Whenever you quote or base your ideas on another person's work, you must acknowledge the source you used. Even when you do not quote directly from another work, if reading that source contributes to the ideas presented in your paper, you must give the author(s) proper credit. If you don't, you are committing plagiarism.
Citations allow you to:
In written academic work, responding to and commenting on the work of scholars shows you've done your research. Adding something of your own shows your critical thinking and understanding of the topic.