Explanation: Amazon designed IAM roles so that your applications can securely make API requests from your instances, without requiring you to manage the security credentials that the applications use. Instead of creating and distributing your AWS credentials, you can delegate permission to make API requests using IAM roles as follows: Create an IAM role. Define which accounts or AWS services can assume the role. Define which API actions and resources the application can use after assuming the role. Specify the role when you launch your instances. Have the application retrieve a set of temporary credentials and use them.
Reference:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-am…