It is important to use citations because there are legal and ethical considerations regarding copyright and plagiarism. Copyright laws were created to promote creativity and protect the work that anyone creates and to make sure they get credit, including financially. When an author/creator's work is used without his/her permission by someone else who is financially profiting from using the work, that is considered copyright infringement.  Plagiarism is different than copyright laws because it involves improperly citing or attributing work to an author/creator.  Plagiarism is unethical, but it is not illegal.  Copy infringement is illegal. 

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 a limited capacity, for instance, to analyze, criticize, or make fun of, you are protected under Fair Use; however, you still need to give credit to the creator/owner. If you are not actually analyzing the work but just using to decorate your presentation, it is not covered under fair use and is your responsibility to verify it is licensed for reuse or to get permission to use it.

It is also important to format documents.  Using citations is one of the most tedious and dreaded parts of writing. Guidelines for formatting academic writing provides consistency for researchers working within a discipline.  While the format itself is not what prevents plagiarism, it is important in terms of developing  credibility.