It's kind of like giving a free demo, really. It's the same principle. You give someone a taste but limit the experience and allow them to unlock everything for a fee.<p>The difference here is that the fees are small and many, whereas in the old days it was just a one-time drop of $49.99, or whatever.<p>This model adapts better to an online, social context because it seems less restrictive, allows participation by everyone, and you can choose how much you want to spend on the experience, and how much of the experience is worth unlocking, and what parts to unlock at what times.<p>I think that it works well for systems like phone apps, where users just want to try something out before dropping much cash on it.