To Show Agreement
One person declaring something to be true can be easily ignored or dismissed. After all, it is only one person's opinion. It may or may not be true. When several people agree that something is true, however, it is not so easy to dismiss.
By including source material in your writing, you tell your reader, in effect, that there is a "chorus" of agreement on your ideas.
That said, be aware that a "chorus" of agreement does not necessarily mean that the "chorus" is right. Citing and documenting the "chorus" simply bolsters the credibility of your argument and gives others the opportunity to research your findings further and come to their own conclusions.
It also indicates that you have done your homework on the subject and that what you have to say can be trusted at least to the extent of your research efforts.