Tomas has written an article for GamesIndustry.biz on the strategy of the current crop of digital distribution stores:
The money is only part of the power of such stores. A strong store also gives its owner lots of options for wider strategic distribution - when your platform has lots of great stuff that people want, it makes it attractive so users will install your client and/or buy your devices. A powerful store also gives you control of a space from where you can lever its audience to do other profitable things. For example, a store owner can choose to (or be paid to) promote certain apps, services or products that suit it. Go to Amazon's webpage and you'll see the Kindle promoted, the retailer using its store front to heavily promote its own hardware. From a biz-dev point of view this sort of horizontal integration makes total sense as once Amazon locks people in to the Kindle, the chances are they'll then use the Kindle's inbuilt store link to buy ebooks so cementing the relationship between the store and customer.