Well, great question. I am not sure if I have a sufficient answer but I will try. A lot of "ranking" is hype esp for a 6th grader. However, I have been following players like J.Mike Reese who is now a 10th grader who started varsity at 9th grade who is a front runner from early on for major d1 recruitment. J.Mike has been ranked since was a sixth grader by the adidas camp and other ranking agencies, and he is well known....just google him. Believe it or not recruiters and scouts are following players from early on...above average players of course..your avg player will prob not get a ranking. So, for above avg players it is a great exposure tool. Believe me someone is paying for the subscription to the ranking agencies. I feel by ninth or tenth grade you want your name out there so that when you coach sends your dvd to some college coach, you want them to at least have heard of your name or your dvd will just get stacked in the corner with all the other unknown named players dvd's that the college coach has received.
Secondly, for the ranked player...just the confidence boost one gets from being marked as a top player nationally is a very positive experience, if the whole phenomenon is channeled correctly by the parents. So, really most of "ranking" is hype but I can say that if you are a parent or a player who is ranked nationally..no matter what...it makes you proud and it is something to build on. One day you should attend a national camp and pretty quickly without asking, you will know who the ranked players are..even though it is hype. Oh and lastly, ranking has nothing to do with AAU. You obtain rankings from national camps. Hope I helped.