The basic purpose of a literature review for a research paper is to help the writer search through the current research on the topic in order to be sure not to belabor or spend too much time on points that have already been made or accepted theories.  It also serves to provide a basis for an informed opinion about a topic and to provide information that will likely form the backbone of the research paper itself.

Because it will expect the writer to properly document the sources in whichever format is appropriate, whether it is MLA or APA or Turabian or something else, it also provides a good practice run for a nice works cited page and gives the writer a chance to make sure their formatting is correct and that they have sufficient source material to produce an informed research paper.

Reading through the current and past literature on a subject also gives the writer a valuable opportunity to consider their position on the topic and to provide viewpoints and information that may contradict or challenge their own understanding which can be invaluable if one wishes to produce an effective position on a particular topic.