It's a pretty neat version of lean startup, in my opinion. Rather than just gathering email addresses from people that might vaguely be interested in your produ

It's a pretty neat version of lean startup, in my opinion. Rather than just gathering email addresses from people that might vaguely be interested in your product, you get cold hard cash from people that are definitely interested enough in your product to buy it. You know for sure you have a market, because they already bought your thing.