What was the significance of the mother's milk in Mad Max Fury Road (2015)? It almost feels like there was a deleted scene that explains it.
Human milk was one of the resources for trade and it most likely was meant for the elite:
"They trade with one another for vital resources. In addition to exporting water the Citadel also exports human milk, as earlier in the film you see humans being milked. The milk is probably exclusive for the elite of society as we only see Immortan Joe's son drinking it and it's valuable enough to Gas Town to trade it for gas. As we don't see any other mammals in the setting, the milk produced by the Citadel is a pretty vital source of protein and nutrients in the wasteland."