Shut-off valves only reduce your water pressure when they are partly or fully closed. For plumbing appliances to operate properly and to protect your plumbing, it is important to maintain the correct water pressure.
Too much pressure can damage pipes and fixtures, but too little pressure can mean faucets and showers take longer to receive water. One way to control water pressure is to use shut-off valves, which reduce or increase flow depending on the position of the valve handle.