Katy Young explains the value of Power BI datamarts:
A datamart can be thought of as a subject-specific data warehouse, often containing data related to one domain or line of business within an organization. Because datamarts are often aligned to a particular business use case, analysts don’t need to write complex queries over large volumes of data as they would against a more traditional data warehouse. This reduces time to insight for organizations and promotes self-service analytics by making structured data accessible to exactly the people who need it.
Read on for more information.