Water needs a bit of a push to keep moving through your pipes. On personal properties, that pressure is supplied by the pump from the well or lake. Cities have a lot of water to push, and so often make use of tall water towers. It’s more efficient to pump water straight up to a central tower, and then let gravity help push it down and then out over the surrounding areas. Some cities make use of pumping stations as well.
The pipes inside your home are designed to withstand pressures from 25 – 80 PSI (pounds per square inch). If there’s too little water pressure, water won’t flow. If there’s too much pressure, it can cause banging noises and puts stress on the joints between pipes, especially as they age. There’s nothing inside your home that controls the pressure – it’s completely reliant on the pressure created by the incoming supply.