I got the idea from actually going through the grad-school application process, speaking with people on admission committees, and to senior graduate students and also looking at data my old department had collected correlating PGRE score with acceptance rate.
Also, you know, you could just read almost any good schools website and look at their requirements and see for yourself the fact that a lot of emphasis is placed on the PGRE.
You, I assume like the OP, are an undergrad with no first-hand experience. So where exactly did you get your ideas? Dare I say?