Let's suppose that you are the leader of a BI project. Let's also suppose that you are following the typical approach to implementing a datawarehouse project with its related reporting tool, the ETL process reading from the source transactional database, and inserting data into your DWH. Following the typical approach you should gather specifications from key users, think of a very robust data model that serves to accomplish those specifications, install all components, extract all the different data that you need from different data sources, validate the integrity of all this data for all fields, define the required reporting, and then you will be able to show to the key user the result. The whole process can have taken months or maybe even years. When you are checking with the key user what has been the result, your user can have changed his mind regarding what he needs or maybe your key user has changed his mind and he has completely different ideas about what to use in his reports.