In real world, end user tools access data from various sources.
As you can see from the following diagram, various tools can access data for various purposes from various sources throughout the information delivery chain.
Some of the reasons for this mixed access are:
Already existing ties of Staging Area with existing data marts.
Production reports being taken out of the normalized area of staging to spare load on Data Warehouse.
Overload on a given data mart of data warehouse to necessitate a parallel, but integrated source.
