You do not need to read everything that has ever been written on your topic, because that may very well be physically impossible, but you must make sure you are familiar with all the key sources in your field that are pertinent to your problem or question.
During these sessions, students can get answers to introduction to the problem, background of study, statement of the problem, purpose of the study, and theoretical framework.
Previous researchers may have also studied what you are about to study and they may have had special problems or difficulties; by reading what they wrote, you may be able to avoid their mistakes.
You may also, in the course of reading over previous research, discover something crucial to your research problem or question that has been overlooked by the main sources you have been exposed to in your coursework.
Literature review is very important because reading the research that others have published on your topic and problem or question will allow you to learn more about your topic and problem or question.
In this way, literature review updates and completes your education, adding to what you have learned in your coursework and discussions with your professors.
You may have to weigh competing theoretical arguments; you may also have to synthesize and evaluate varied strands of research conducted on different populations, different samples, and using different measures and research methods.
Research that is published while you are writing your dissertation may add to what is known about your topic or provide additional evidence for or against one or another theory.
You may even have access to research that is so recent it has not been published yet.
In all these ways, doing a literature review is not only crucial to writing a dissertation, it is also giving you experience with the ongoing process of new knowledge and discovery that is taking place in your field.