Choosing Sources to Establish Credibility
The main reason writers include sources in their work is to establish credibility with their audience. Credibility is the level of trustworthiness and authority that a reader perceives a writer has on a subject and is one of the key characteristics of effective writing, particularly argumentative writing.
Without credibility a writer's ideas are easily dismissed. Including sources in your writing indicates that your opinions are based on more than a personal or surface knowledge of the subject. It shows that others agree with your ideas, that experts in the field corroborate your reasoning and that there is hard evidence to support your opinion.