In Fate/Stay Night and Fate/Zero, Irisviel and Iliasviel were used to summon the grail. What would happen, if the Einzberns hadn't created Irisviel in the first place? Could the grail still appear? Would another family need to create a homunculus? Would that even be possible, due to the fact that the grail was first summoned with the use of Justeaze Lizrich von Einzbern?
Up until the end of the third Grail War, the vessel for the sought-after Grail, known as the Lesser Grail, was a literally a cup.
According to the Type Moon Wiki:
However, during the events of the third War, it was damaged in battle and the ceremony was interrupted; for that reason, it had to be made as something that could manage itself and take care of itself.
During the Fourth War, Irisviel von Einzbern was a homunculus forged by the Einzbern family to be the physical embodiment of the Lesser Grail.
In other words a Einzbern homunculus has to exist in order to serve as a vessel to connecto the Great Grail, it stores the souls of the heroic spirits after they're killed.
If everything goes according to plan, the Lesser Grail will be able to summon the Great Grail and use the power of the heroic spirits and the Third Magic to create a gateway to the Root. Afterwards, the Grail can technically grant wishes, due to all the leftover prana.