It's been a while on-line services spread all over the Internet. Mails, instant messaging, file storage... you name it! For now, it's quite hard to monetize these services. Web surfers are not used to paying for anything on-line. Therefore developers had often to use advertising.
But advertising isn't very efficient. Therefore the market had to adapt. The answer might be applications stores. Applications stores smooth on-line payment so that it will be easier to pay for the services you consume.