To understand SRP, it is helpful to understand the role of the AEM repository (OAK) in an AEM community site.
Java Content Repository (JCR)
This standard defines a data model and application programming interface (JCR API) for content repositories. It combines characteristics of conventional file systems with those of relational databases, and adds a number of additional features that content applications often need.
One implementation of JCR is the AEM repository, OAK.
Apache Jackrabbit Oak (OAK)
OAK is an implementation of JCR 2.0 that is a data storage system specifically designed for content-centric applications. It is a type of hierarchical database designed for unstructured and semi-structured data. The repository stores not only the user-facing content but also all code, templates and internal data used by the application. The UI for accessing content is CRXDE Lite.
Both JCR and OAK are typically used to refer to the AEM repository.
After developing site content in the private author environment, it must by copied to the public publish environment. This is often done through an operation called replication. This happens under control of the author/developer/administrator.
For UGC, the content is entered by registered site visitors (community members) in the public publish environment. This happens randomly.
For purposes of management and reporting, it is useful to have access to UGC from the private author environment. With SRP, access to UGC from author is more consistent and performant as reverse replication from publish to author is not necessary.