サービスリンクロールに対応してるサービスならRoleArnは不要だが、EMRみたいにサービスリンクロール未対応ならRoleArnが必要。
// This parameter is required for services that do not support service-linked // roles (such as Amazon EMR), and it must specify the ARN of an IAM role that // allows Application Auto Scaling to modify the scalable target on your behalf. If // the service supports service-linked roles, Application Auto Scaling uses a // service-linked role, which it creates if it does not yet exist. For more // information, see Application Auto Scaling IAM roles (https://docs.aws.amazon.com/autoscaling/application/userguide/security_iam_service-with-iam.html#security_iam_service-with-iam-roles) // .
