When we cite our sources we are acting ethically and courteously.
Ethical research practices
When you provide attribution for ideas, quotations, and evidence that you have gathered through reading articles, studying reports, looking at images, listening to music, reading primary and secondary works you are acknowledging, providing credit for, the work of others that you have relied on to build your own arguments and conclusions.
Courtesy to your readers
Your readers will want to follow the path you followed to get to your conclusions. Proper citation lays out a clear map for them to follow. Just as you may follow up on citations that you find in the bibliography to a book or article that you've read. Your readers will also want to do this. Your carefully and accurately applied citations and bibliographies will make this easy.
Barbier, Edward B. Cash Crops, Food Crops and Agricultural Sustainability. International Institute for Environment and Development, 1987, http://www.jstor.org/stable/resrep01650.