Webpages expire, books and articles get lost, photographs and films degrade. Citations are necessary to assure that the next researcher can find the same information through different means.
Citing is important for your scholarly credibility, and for building on previous research. You may have a good idea, but simply stating it does not make it true or believable. Give your ideas validity and support by citing established authors.
Avoiding plagiarism is key to academic honesty. Nothing is worse to an author than discovering their hard work has been stolen and claimed as original by someone else. Citations give authors their due credit.
The Writing Center at Purdue University has instructions and examples on APA and MLA citation styles. Check out their Online Writing Lab (OWL).
The Excelsior College Online Writing Lab provides extensive information on the writing process, evaluating research, and tutorials on building citations in APA, MLA, and Chicago style(s).