In order to make a decision about a particular subject area, you should analyze all the required data that is relevant to the subject area. If the data is stored in a variety of data sources, importing the data from different data sources has to be done. If all the data is only in one data source, only the data needs to be imported from the required tables for the subject and then various types of analysis can be done.
The reason why users need to import data from different data sources is that they would then have an ample amount of data when they need to make any analysis. Another generic reason would be to cross-analyze data from different business systems such as Customer Relationship Management (CRM) and Campaign Management System (CMS). Data sourcing from only one source wouldn't be as sophisticated as an analysis done from different data sources as the amount of data from which the analysis was done for multisourced data is more detailed...