A literature review helps you establish your “authority” on the topic. As a student in social science, you may not be an authority, much less an expert. Yet, by listing an extensive literature review you have done, you are claiming you did your homework, which will help establish a rapport with the reader -- that you have the facts about what you are talking about. Thus, it's important for you to list all the books you have read, articles you have evaluated, interviews and experiments you have conducted and even emails you have exchanged with the experts. Without such a thorough literature review, your reader will not be able to trust your authenticity.
Similarly, by writing an extensive literature review, you can avoid repeating research someone else has already done. As you evaluate other scholarship performed on your topic, you can make sure you're not rehashing old ideas. You could be accused of committing accidental plagiarism, although you may not intentionally have stolen anything from anyone. In the process of compiling a literature review, you will be able to examine what kind of scholarship is available on your topic or how your new research is different from what has previously been done.
As you compile your literature review, you will summarize, analyze and synthesize outside sources in your own words, thus differentiating the chaff from the grain by placing individual works in the context of your research, helping you determine the relevancy of each work. This process will sharpen your understanding of the research issue you are working on. The review will also describe the relationship among various scholarships, identifying any gaps or seeming contradictions from previous works. Such identification and delineation will further help you explain and establish why there is a need to do more research on your topic in the future.
A literature review can be written in many different ways, such as argumentative review, integrative review, historical review, methodological review, systematic review and theoretical review. An argumentative review is written to pose an opposing view to an existing position; an integrative review consists of reviews and criticism on a given topic to introduce a need for a new research. A historical review traces all the scholarship that has been done on a given topic chronologically, while a methodological review examines the methods -- data collection, analysis and interpretation -- of the research, for example. In short, a literature review gives the reader a quick comprehensive summary of the topic, scope and scholarship, past and present, providing a sense of direction of the research.