Additionally, you'll know what the shortcomings of previous studies were and have a better idea on the potential gaps of literature. These will give you the advantage of building original hypotheses. 

Dear Bushra ,

I sent you a pdf about your question.

I hope it is useful.

I only find it. not write it.

I think it helps you develop robust knowledge of the area you are studying. By it you are able to identify relevant baseline theories surrounding your topic, you are also able to know earlier results on the topic or related topics, and this provides you with the roadmap to navigate your own research . It eventually makes your discussion of findings very convincing because you are backed with sufficient literature that either corroborates or disagrees with your result. 

Justin says it well. Another way to look at it is in terms of journalism.

"Bad Journalism" is reporting without context, background, or consideration of what may be considered normal versus unusual or rare. "Bad Research" is much the same. The use of the literature is part of the metadata of any research work and allows building not strictly of consensus, but building of connections and pathways to deeper and/or new knowledge.

I echo Justin and Paul's comments. As a doctoral student preparing for your dissertation, you are expected to be the expert on the topic at hand. You would not develop the expertise without performing an extensive literature review. A systematic review gives you the framework for conducting such a review so that you capture the relevant historical record of the topic.

In the end, you will likely know more about the relevant research than your doctoral committee, and when you stand up to defend your dissertation, you must be able to respond to each question as an expert in the field.

Additionally, you'll know what the shortcomings of previous studies were and have a better idea on the potential gaps of literature. These will give you the advantage of building original hypotheses. 

Thanks everyone

Just a quick additional thought...

Even in specialized areas it is important to cross disciplinary boundaries to see if there is also relevant literature (and analogies) in other fields that might be pertinent. Sharing across disciplines (contextually and conceptually) has always proven to be insightful as it allows for new perspectives you may not have considered within your own disciplinary literature.

On the other hand, is there any drawback/weakness for syst. literature revs.?

In my opinion, yes, there are drawbacks based upon how the review is accomplished. For example, if only a limited set of journals (or sources) are used or if they are only used in a very selective manner. This can lead to biased reporting and could then impact the research planned by an investigator.

In a nutshell, a literature review enables your study to address the gap which adds light to your statement of the problem. The past studies in the literature will add a dimension to your methodology too.

PhD is a research taking from a couple of years to quite more than that, and from a couple of publications to sometimes near a dozen of them, or more. Being updated and refreshed by the latest results in your scientific domain is a professional necessity at any time.

Nevertheless, it is especially important by the time when the achieved results are being wrapped up into a dissertation. A detailed review done by that time allows to find that Ariadne Thread through the labyrinth  of your achievements, thoughts, ideas, and approaches. In short, it is a possibility to convey yourself as a ripen scientist.    

We have recently published guidelines for conducting systematic literature reviews. We used the example of SCM, but it can be adapted to other disciplines. You can find our article on my ResearchGate page. Christian F. Durach, Joakim Kembro & Andreas Wieland (2017), A new paradigm for systematic literature reviews in supply chain management, Journal of Supply Chain Management, Vol. 53, Issue 4.

@Andreas, We recently publish the approach for systematic review. 

A protocol for a systematic review to identify allergenic tree nuts and the molecules responsible for their allergenic properties. 

DOI: https://doi.org/10.1016/j.fct.2017.06.004