The GRE Physics score is used differently at different institutions but, whatever the particulars for admission in any given graduate program a high score on the GRE Physics exam is a major benefit. Another advantage of a high GRE score is that it can often compensate for a somewhat weak grade point average. For example statements by the applicant such as , I took too big a course load'. I did not get along with some of the professors', I had a bad year my Junior year', etc. The GRE Physics exam is difficult , and the competition is strong.
Adequate preparation is critical for a good performance.
Feedback from former students is quite clear on the GREs. Form a study pair or group and go over as many example GRE exams as you can. Schedule regular study sessions and stick to them. This relieves some of the fatigue you will experience in taking the exams. At the absolute latest, take the GRE exam by the October exam date during the Fall semester of the Senior year.