A citation tells readers that the information provided originated from a different source. It also provides the information needed to locate the source if needed.
Give your paper credibility
Often, your opinion is not enough. You need to back up arguments with research. Think of a criminal court case; lawyers need evidence in order to have a strong argument. Rely solely on their opinion and they will almost surely lose. The same goes for you. When reading a research paper professors wants to claims backed up with reliable information.
Give others credit for their work
Citing a source properly gives appropriate credit to the person who created the work you’re using. It also makes clear what is your work and what is theirs. Writing books, journal articles, and other content is hard work. Properly citing sources gives credit where it’s due.
Plagiarism is using someone else’s words or ideas without giving them credit. In other words, taking personal credit for the work of other people. Being caught plagiarizing can result in disciplinary action by your professor or the university. Professionals who plagiarize can be fired. To avoid plagiarism, cite correctly.
Because your professor says you have to
There are a lot of reasons why you should or have to cite, but at the end of the day you need to cite because your professor often says you have to. Not citing sources in your paper can get you in trouble as described above, but it can also cost you points on your assignment.