Ask a bank what your bank account balance is today and they can tell you. But ask them what your average balance has been over the past twelve months and they have no idea. The essence of data warehousing is integration and history. Integration is achieved by the messy task of going back into older legacy systems and pulling out data that was a by-product of transaction processing, and converting and integrating that data. Integrating old legacy data is a dirty, thankless task that nobody wants to undertake, but the rewards of integration are worth the time and effort.
Historical data is achieved by organizing and collecting the integrated data over time.
Data is time-stamped and stored at the detailed level. Once an organization has a carefully crafted collection of integrated detailed historical data, it is in a position of great strength. The data can be organized a certain way on one day and another way the next. Marketing can look at customers by state or by month, Sales can look at sales transactions per day, and Accounting can look at closed business by country or by quarter - all from the same store of data . So the first tremendous business value that a data warehouse brings is the ability to look at data any way that is useful.