Why use PBI Datamart when SQL Data Warehouse Datamart already being leveraged?

Hi All!

Apologies if this is documented in a nice article or other post, but was unable to find it.

If I'm already leveraging a Datamart, upstream in a SQL database, what are the use-cases of having one between that DW Datamart and the Dataflows that I already use flowing from that external Datamart? Is there something significantly value-add in the PBI Datamart that would benefit from shoe-horning in there?