Alberto Ferrari has another good use for calculation groups:
This technique works just fine; it has the disadvantage of creating many measures, one for each combination of relationship to activate and base measure. Another solution is to create a calculation group that changes the active relationship of the selected measure. Doing this, you create one calculation item for each relationship and the user chooses the relationship to activate using a slicer or a report filter.
But read on for the calculation group solution, which is a clever way of deferring which relationship you care about until the user selects it.