There are several reasons why students are asked to cite sources in academic papers. One is that it is something done in professional writing and thus it's good practice. Another reason is so the a reader can evaluate the credibility of your research. If, for example, someone is looking for advice about a medical condition, an article written by a doctor and published in New England Journal of Medicine is far more likely to be trustworthy than something written by someone trying to make a profit by selling untested herbal remedies. Another reason to attribute work is to avoid plagiarizing -- to copy someone's ideas without giving them credit is a form of stealing.