The manifold is up high so gravity can do it's job (push water down and out the 1/4" lines without a pump). I have found it's easier to equalize the pressure to each plant site using gravity vs. a pump, but some adjustment is still needed (hence the 1/4" shut off valves at each plant site). You are correct that without the 1/4" shutoffs to adjust flow the lower plants would get a lot more water than the upper level plants. With them I'm able to adjust flow so that each plant gets the same amount of water.