Behind citation is a great legal and ethical consideration of copyright and plagiarism. Copyright was created to promote creativity and protect the work that anyone creates and to make sure they get credit. Even this website is under copyright protection because as I type this I am creating it. Plagiarism is one form of copyright infringement.
It is always your responsibility to give proper attribution, or credit, to your sources and to make sure you have permission to to use a source. If you are using a copyrighted work in your school work in order to analyze or critique it (i.e. analyzing the composition of a camera angles in a film or the use of color in a painting) then it would fall under Fair Use, but you still need to give credit to the creator/owner.
Formatting documents and citations is one of the most tedious and dreaded parts of writing for students AND professional writers. Guidelines for formatting and citations provides consistency and easy of use for researchers working within a discipline. While the format itself is not what prevents plagiarism, it is important in asserting credibility and finding success in your academic career.