It's hard to say without knowing the app, but my first guess would be for storing files in S3 or pushing a message into a SQS queue.

AWS supports temporary access keys, and one of the recommended solutions is to have an API which generates temporary credentials for a specific task that will expire shortly after.

Edit:

Heres a link http://docs.aws.amazon.com/STS/latest/UsingSTS/CreatingSessi...