Mass flow is just the idea that substances are transported from one destination to another.
For example ,the bloodstream carrying useful substances like oxygen and glucose but also waste products like carbon dioxide.
you should always include mass flow as a part of your answer when the question asks
"Why do humans and animals need a heart and circulatory system?"
So in this case you would say Mass flow would important in order to deliver essential substances like Oxygen.
Without a heart and circulatory system simple diffusion will not be enough to meet the demands of the organism.
Whereas a heart and circulatory would pump blood continuously in mass flow so oxygen is always readily available.
(I hope that all made sense >.<)