Besides the obvious reason for students -- because it is assigned! -- a literature review helps you explore the research that has come before you, to see how your research question has (or has not) already been addressed.
core research in the field
experts in the subject area
methodology you may want to use (or avoid)
gaps in knowledge -- or where your research would fit in
It Also Helps You:
Publish and share your findings
Justify requests for grants and other funding
Identify best practices to inform practice
Set wider context for a program evaluation
Compile information to support community organizing
Great brief overview, from NCSU
Want To Know More?
Literature Review and Research Design by Dave HarrisDesigning a research project is possibly the most difficult task a dissertation writer faces. It is fraught with uncertainty: what is the best subject? What is the best method? For every answer found, there are often multiple subsequent questions, so it's easy to get lost in theoretical debates and buried under a mountain of literature. This book looks at literature review in the process of research design, and how to develop a research practice that will build skills in reading and writing about research literature--skills that remain valuable in both academic and professional careers. Literature review is approached as a process of engaging with the discourse of scholarly communities that will help graduate researchers refine, define, and express their own scholarly vision and voice. This orientation on research as an exploratory practice, rather than merely a series of predetermined steps in a systematic method, allows the researcher to deal with the uncertainties and changes that come with learning new ideas and new perspectives. The focus on the practical elements of research design make this book an invaluable resource for graduate students writing dissertations. Practicing research allows room for experiment, error, and learning, ultimately helping graduate researchers use the literature effectively to build a solid scholarly foundation for their dissertation research project.