Membership cards are cool. You can carry them in your wallet. You can show them to friends. Maybe you can use them to get into events or get discounts on relevant products?
But, if you're running an online community, you probably don't want to give a membership card to every member that joins. That could get expensive, right?
So why not give a membership card to members after 100 topic posts? Or 50000 karma points? Why not giving the membership card some real meaning.
It’s not cheap, but it's a simple way of increasing engagement.