Information is extracted from heterogeneous sources as it is generated or updated. The information is then translated into a common data model and integrated with exisiting data at the warehouse. When a user query is submitted to the warehouse, the needed information is already there, with inconsistencies and differences already resolved. This makes it much easier and more efficient to run queries over data that originally came from different sources. End users can use a single data model and query language.
System design becomes simpler.
For example, there is no need to perform query optimization over heterogeneous sources, a very difficult problem faced by other approaches. Information sources may be unreliable and may purge data.