Controls the flow of your water and pump so you set and a desired flow rate of water that works for you and with an inline tap on your pole hose or end of microbore when you turn the flow of water off the pump will stop pumping .
Otherwise you will be running a pump flat out with no way of stopping it apart from walking back to van and turning it off use an inline tap without an electronic pump flow controller and the connections will blow off and you will have water all over the back off your van .
I tried it once and within a few seconds water was pouring out the back of my truck a 100psi pump is to point way to over the top for WFP use as we just dont need a pump that powerful for window cleaning , I run mine at about 1/3 of its capacity .