The main purpose in using a water pressure is to regulate incoming water supply from the source. Water pressure differs from campground to campground. The pex tubing in our rigs has a 55 psi max capacity (per the specification). Many campgrounds supply water pressure in excess of the capacity. If its too high, it can damage tubing but more likely cause leaks at connections.
Also, if you have a black tank flush, it has a back flow preventer. This is located in various places in the rig. In most it is in the bathroom cabinet under the sink. It is a back plastic device connected to two white pex tubes. It prevents water from the black tank flush backfeeding into the hose you are using for flushing the black tank. It doesn't take too much pressure over 65 PSI to cause the back flow preventer to fail. When this happens, it leaks...a lot. I speak from first hand experience.
So it is wise to regulate the water pressure for both the water system and black tank flush. BTW we also use a two tank filter system for incoming water as well as a 3 filter DI system for drinking water as the definition of potable water differs from place to place. Best Regards.