Take for instance a third party music app. A developer can now build in the ability for a user to purchase an MP3 or CD directly through Amazon, where the user most likely already has account info set up. Through the Amazon Affiliate program, that developer would now receive a part of the sale and they didn't have to go through the trouble of building a mobile storefront. It's a win-win.
More info regarding the SDK and API's can be found on the Amazon Mobile Associate page.