Data Warehouses are central repositories of integrated data from one or more disparate sources. They store current and historical data which allow creation of actionable insights to be consumed throughout the enterprise.
They are the foundation for a successful BI program. The concept is simple; create a central location and permanent storage space to house a deep data pool that can be sliced and diced in an infinite number of ways to support desired analysis, reporting and other BI functions.
In-house Data Warehouse design, development, infrastructure and maintenance can be very expensive and inefficient due to the need to have scale based around peak demand.