Systematic review is the first step in answering any research/policy/practice question. The only way to find the answer is to find and check all the relevant answers and literature systematically, neither randomly, nor selectively! There are more than 48 types of literature review with some ‘systematic’ components. The type of the review you choose depends on your needs, time, and resources.
Systematic review is the only way to assure you that you are not missing anything and your work has novelty and is not a duplicate. Either it is your personal work or a grant application, it starts with a systematic review, continues with systematic review and ends with it.
Above following diagram, shows the steps involved in systematic reviewing process, however, not all literature reviews follow all these steps and people usually tailor the process to fit their purpose.