As much as you may know about a subject, to discuss it still presents a reader with one source. Writing a research paper requires you to broaden your intellect by seeking out sources of information that you can use to learn more about a topic. In the process of researching you are also developing skills in citing the work you have found and giving credit where credit is due. Overall, building research skills by citing multiple sources adds credibility to your work by showing the rigor with which you seek to develop your paper.
Writing a research paper also forces you to see multiple sides of an argument. As your resources grow you will come to find that there are many ways to view your subject matter. In order to write the paper you will often have to choose a persuasive argument and find sources that support your perspective. You cannot just write your opinions. The act of finding, reading and critically synthesizing the information into your argument requires skill and a expanding intellect. The more you write and work on research papers the more your critical thinking skills will grow.
There once was a time when the only way you could find information was to go to the library and read an established published work on a given subject. Today the Internet has opened the door to more convenient ways to gain information, but it has also allowed students to forget the need to verify and critically source their work. You cannot simply say, "I found it on the Internet and therefore it must be true." Writing a research paper pushes you to find multiple corroborative sources and to be more discerning about the information you find.
The research paper is a rite of passage for students. It is not a frivolous exercise in writing -- it is an extended purposeful test of your ability. You should be able to write at length and show fluid and cohesive thoughts, building an appropriate and well-conceived argument. Many classes will have a research paper as their final and largest project of the semester. Here you are responsible for using the materials of the course and creating original and independent work to show that you are prepared to move to the next level of academia.