Why should I use amazon Simple Notification Service (SNS) ?

With the release of Reduced Redundancy Storage (RRS) one of the more common feature requests is to provide a notification message when Amazon S3 detects that an RRS object has been lost. Customers can now have notifications sent to an Amazon Simple Notification Service topic when Amazon S3 detects that an RRS object is lost. Notifications are enabled via a bucket sub-resource (like versioning or server access logging).

RRS is a new storage option within Amazon S3 that enables customers to reduce their costs by storing non-critical, reproducible data at lower levels of redundancy than Amazon S3's standard storage. Amazon S3 and RRS both store data in multiple facilities and on multiple devices, but with RRS, data is replicated fewer times, so the cost is less. Amazon S3 standard storage is designed to provide 99.999999999% durability and to sustain the concurrent loss of data in two facilities, while RRS is designed to provide 99.99% durability and to sustain the loss of data in a single facility.

So RRS is cheaper in cost  and have facility to get notification by amazon when RRS object is lost.