At some point in your academic tenure, you will be asked to write a research paper.
A research paper combines two fundamental elements of academic writing: (a) analyzing and synthesizing outside evidence and (b) providing your own evidence-based interpretation or argument about a given topic within your field of study.
A literature review can be a precursor to the introduction of a research paper, or it can be an entire paper in itself, acting as the first stage of large research projects and allowing the supervisor to ascertain that the student is on the correct path.
A literature review is a critical and in depth evaluation of previous research.
Evaluating the credibility of sources is one of the most difficult aspects of a literature review, especially with the ease of finding information on the internet.
The only real way to evaluate is through experience, but there are luckily a few tricks for evaluating information quickly and accurately. Google does not distinguish or judge the quality of results, only how search engine friendly a paper is.
It is a summary and synopsis of a particular area of research, allowing anybody reading the paper to establish why you are pursuing this particular research.
A good literature review expands on the reasons behind selecting a particular research question.
The fact that a researcher is undertaking the research program speaks for its importance, and an educated reader may well be insulted that they are not allowed to judge the importance for themselves.
They want to be re-assured that it is a serious paper, not a pseudo-scientific sales advertisement.