As Al said, the pressure can spike unpredictably.
Also, the reason I have a sediment filter before my regulator is you never know what is going to come out of the line. In the larger RV resorts, some water lines that are on end runs accumulate sediments. I've turned on some spigots prior to hooking up and seen brown water until for at least a gallon or two. That's stuff I don't want to go through the regulator or get to my HW heater and carbon filter at my kitchen sink.If seen my filter turn dark brown after two weeks of use.
And that was a park that was on city water not a well.