The central reason to include secondary sources in an essay is to support your ideas. Without sources, the essay shows only your interpretation of the topic. With the added support, you show that other scholars share your perspective. You bolster your argument by demonstrating that it is accepted in the field.
Another reason to include research is to show that you have interacted with the scholarly community surrounding your topic. It is relatively simple to write your personal opinions or experiences about a subject. However, in an academic environment, you want to show that you have knowledge of what is being said in the relevant journals and publications and that you have ideas to add to the conversation. You may agree or disagree with other critics as you engage in dialogue with them.
Documenting a full range of information about each of the sources in a source list is a vital step to supporting your ideas and engaging with the academic community. Others in the field can use your citation and documentation information to access the same sources you used to further their research and evaluate your ideas.
Sometimes an essay will use both primary and secondary sources. A primary source was written or created at the time being studied. For example, if a writer were researching Helen Keller, letters or journals written by her would be primary sources. In literary analysis, the piece of literature itself is the primary source. Secondary sources are written about the primary sources, usually at a later date. Biographies of Helen Keller and critical articles about literature are examples of secondary sources. Both primary and secondary sources are listed on the same source list.