Excel offers a few things in most scenarios, primarily because that any enforced structure always implies lost flexibility, and one of the issues often quoted with Excel is the lack of enforcement of using the right structures and data sources.

A simple one: If I have been asked to do a report, but it turns out I don't have the correct access rights to some data, my manager can simply send it to me in an Excel sheet, or he can log an issue with IT to change the acess rights to a specific key/dimension/hierarchy.

The 'one true data source' move often correspond to tighter access controls when good auditing really would have been the correct solution, hence the above situation is fairly common.