A reporting application is a target application on which you can report on and aggregate Planning data. The primary use cases:
The reporting application contains data from any source, such as a data warehouse. You want to push new Planning data to it and report on the data.
You want to report on Smart Lists in Planning, which you cannot do in Planning.
You can map dimensions between a source Planning application and a reporting application to enable:
Reporting on Planning data in a reporting application
Aggregating and querying on Smart Lists, which are converted to regular dimensions in the reporting application
Linking Planning data to multiple reporting applications for various consolidations