They get sent out to people who play. Since I only Dm, I get the DM benefits (a LOT of free miniatures).
The cards basically allow you to do extra stuff. The concept is that the more that you play (and indicate that online) the more options you have open to you. It's the benefits of not being a novice ("Hey, you're stuck with the cleric") or "Hey, you get to play the elite Psion because you're an experienced player."
Since, I've been playing and DM extensively, but not so much in the past year or two..I might be stuck with the cleric :|
Our HOME group plans on playing this for more than two reasons:
1. The adventures are pre-written (less work and DM's can tag-team and play on the off nights). This is a HUGE issue, because we all have jobs and lives we need to live out when we're not gaming (unlike the good ol' days when we could just create our own adventures and not have social services taking our kids away for neglect..

2. If we miss any adventures at home, we can head out to a game day to play.
3. There are consistent rules (we like that as players)
4. They allow psionics (unlike the 'other' campaigns)
5. You can home-play big campaigns
6. ..there's no Arcana Unearthed worldwide campaign
7. We can share our experiences with lots of other people world-wide
8. Level advancement is consistent
etc.
jh
..