Suppose you want to compare one customer with all the customers. You select one customer per pager and want to have a dataview with the sales of the one customer directly comparing it with all of them. You add two cubes, one with the sales for that customer and another with the sales of all customers. That second cube could have a 'refer to' to a parent entity of the customers. That parent entity encompasses all of the customers and would effectively show the result unfiltered.
In the cube query language MDX there are automatic parents for all hierarchies. Those parents are designated [hierarchy].[all]
If Board entities had an automatic .all parent, dataviews and procedures would have a more powerfull wayto modify the screen select.
Those entities can of course be created manually but that would entail tedious changes to a current hierarchy. Having an automatic element would mean easier database management.