Because of the level of authority and credibility evident in scholarly sources they contribute a great deal to the overall quality of your papers. Use of scholarly sources is an expected attribute of academic course work.
In all disciplines, knowledge is built by responding to the ideas and discoveries of those who came before us. Scholarly journal articles are unique in that they require authors to document and make verifiable the sources of the facts, ideas, and methods they used to arrive at their insights and conclusions. Scholarly articles also strive to identify and discuss the merits of alternative explanations and viewpoints for the positions they espouse. This makes it easier to assess the truth, as well as the strengths and weaknesses, of the claims made in a paper. This is the case for those with knowledge of a subject (for example, your professor), as well as for those just beginning to learn about a subject (for example, you).
As you know, anyone can say just about anything in articles posted on the web. While you might agree with the conclusions of a paper found on the web, you are often not given the chain of evidence you need to assess the truth of those conclusions. Likewise, articles published in popular magazines, while they provide information and opinions, are not required to document evidence that either supports or negates their conclusions. Scholarly journal articles, unlike web-based or popular magazine articles, are designed and structured to provide the elements necessary to most thoroughly evaluate the validity and truth of an author's position.