I don't know exactly, but at D2 and D3 all they did was confuse me. When I first started I would look at the top recruiting classes and try to figure out what was so good about them, and try to model my classes after them the first couple of seasons. At the D2 and D3 levels they are completely worthless, probably not even worth the time to open the email.
It's my understanding that the rating system is a remnant from when potentials didn't exist, so only the players current ratings are taken into account. That should tell you all you need to know about the efficacy of the rankings right there.