What is the advantage of having iRODS or any other low level storage management?
What are its benefits?
When should iRODS be used?

Don’t think of iRODS as low level storage management.

iRODS is really the only platform for policy managed data preservation. It does indeed virtualize storage, providing a global, logical namespace over heterogeneous types of storage, but it also allows you to enforce preservation policies at each storage location, no matter what client or access method is used. It also provides a global metadata catalog that is automatically maintained and reflects the application of your preservation policies, allowing audit and verification of your preservation policies.

iRODS is developing a powerful metadata management capability, allowing pluggable indexing and query capability that allow synchronization with external indices (e.g. Elastic Search, MAUI, Jena triple store).

With the pluggable rule engine and asynchronous messaging architecture, it becomes rather straightforward to generate audit and provenance metadata that will track every single (pre- and post-) operation on your data, including any plugins you may develop or utilize.

iRODS is middleware, rather than a prepackaged solution. This middleware supports plugins and configurable policies at all points, so you are not limited by a pre-defined set of tools. iRODS also can be connected to wide range of preservation, computation, and enterprise services, and can manage large amounts of data (both in number of objects and size of those objects), and efficiently move and manage data using high performance protocols, including third party data transfer protocols.

iRODS is built to support federation, so that your preservation environment may share data with other institutions or organizations while remaining under your own audit and policy control. Many organizations are doing this for many millions of objects, many thousands of users, and with a large range of object sizes.