Webpages expire, books and articles get lost, photographs and films degrade. Citations are necessary in order to assure that the next person would be able to access the same information through different means.
Citing is also important for credibility and building on 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.
To avoid plagiarism - 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.