There are a lot of reasons to take the Physics GRE. The obvious one is for admission the way you mentioned. But some people take it a year ahead of time in April, and then retake it in November for the December admission deadline. There is nothing wrong with taking it twice or three times.
Other people take it once they are already in a graduate program, because the department requires it for various reasons.
And finally, a dirty little truth is that a few physics phds take the test because they enjoy it, it keeps them in "thinking trim" and it's like a game for them to see how high they can get their score. Of course it screws up the percentile for all the normal students, but anyone is allowed to take it.
Interpretation of the score has changed over the last several years, because students in China and Russia (the super students) take it now, and often get near-perfect scores. This has strongly influenced the percentile. A 35 or 40 percentile is a respectable score.
I think one of the strongest considerations for entry into a lot of programs is some kind of work-internship experience with a research group. I've seen firsthand that some students with 92 percentile GRE scores are flakey sometimes blow off their TA and grading duties, and are often not valuable additions to a typical research group. Physics departments are becoming more 'corporate,' and are looking to create strong research and teaching teams, and less interested in finding potentially brilliant individuals.
Of course, if you want to go to Princeton, you'll still need excellent grades and a good physics gre score, but even there, they want to continue to build a strong program.
And ironically, I've noticed that the real "high-fidelity" physics schools tend to hire a lot of professors from "poxy" colleges in the boondocks, while the state schools tend to hire professors who were trained at the "hi-fi" schools. I'm not sure what all this means, but it might be that the most respected programs are looking for the brightest researchers and best teachers, regardless where they trained, while the state schools want the high-class credentials.