Think of a coral as a being with its nose, ass and mouth in the same place. The coral eats, shits and breathes while building a massive comunity building with other beings who also need to eat, **** and breathe. They are quite delicate creatures. The whole point of the water flow is to transport oxygen so it can breathe, food so it can eat, and the flow will flush away the crap. So now we have one happy polyp. This being like us prefers a gentle current all around it and not a jet that will make us collapse. Depending on the type of coral and density it will need more or less flow to reach the inner parts of the colony.