When conducting research in college, it is essential that you are using legitimate, scholarly resources.
Well for one, because your professors said so. If you want a good grade, you need to follow the requirements of your assignment laid out by your professor.
More specifically, because these resources are written by experts in the field and reviewed by other experts in something called the peer review process. The only way to trust that the information you are presenting in your paper is legitimate is to cite information that is vouched for by experts. Make sense?