Well, out of curiosity i signed up, it tracks your most played games and gives stats on them.
There is also a reward point scheme, these seem to be septate from Microsoft points, in the same way that Tesco clubcard points are not real money. Other than that, you seem to be able to earn points based on renewing your membership to gold, or subscribing to sky player, oh and doing a survey about your "media" plans for summer. Right now there is no actual information on the types of rewards other than points.
So what is it for?
As far as I can see it's a glorified Clubcard, Nectar Point etc reward scheme, but with intangible ways of collecting points, and also for undisclosed rewards.
It all seems a bit bollocks for now, especially since the "rewards" will only count from the point you signed up for it (being a XBLA subscriber since 2007 this bugs me)
Ah well lets see how it pans out, probably best to sign up for this sooner rather than later.
Love and "media" plans