Researching your topic and citing your sources helps lend credibility to your paper. It demonstrates that you sought out similar information from experts in the field, which makes what you're writing more believable.
Because research backs up your ideas with facts from established experts in the field, it's an industry standard. If you hope to publish in a scholarly journal, you'll have to conduct thorough research and cite it correctly to be taken seriously.
Research gives you a broader understanding of your topic so you can speak with a more authoritative, informed voice. It allows you to understand both sides of an argument and any additional factors that contribute to your thesis statement.
Many instructors assign research papers so students can develop or strengthen their research skills. In that respect, research is important because it makes you better and finding information and backing up your ideas with facts.
Research helps prevent plagiarism. You may think you have a groundbreaking idea only to discover that a famous scientist already had this idea and wrote on it extensively. Taking credit for this idea, even though you thought it was original can still constitute academic dishonesty. Research helps you use that scientist's work to build on and add credibility to your idea.