Why do we cite our sources?
The first answer that comes to mind is to avoid plagiarism. We know that we must give credit to others for their ideas. And if we ever get something published, we would expect that others would give us credit where credit was due. It is part of the tradition of scholarship.
The second reason that we cite our sources, expecially as historians, is so other researchers can follow our trail of information, so they can find our sources and read more for their own research. This is how we find more good stuff when we read scholarly books and articles...we can read what the authors read and maybe come up with our own new conclusions.