The idea should be that you don't have to reinvent how to authenticate users (client developer and end user), apply different quotas, charge money, detect abuse

The idea should be that you don't have to reinvent how to authenticate users (client developer and end user), apply different quotas, charge money, detect abuse, log any of that, nor a number of other things that are common to all APIs. Not sure how much of that does Amazon accomplish, nor to what success.