If water pressure and volume are proportional as another question and answer seem to suggest, why can't I reduce my home water pressure by just partially closing the main water valve to my home water supply? Will this not reduce pressure as well as volume? A plumber told me doing so reduces the volume but not the pressure, and that I need a pressure reducing valve. Was he right, and if so what does the pressure reducing valve do that partially closing the main water valve doesn't do?
Choking off your supply by closing down the inlet valve flow area will only reduce pressure downstream of the valve as long as you have flow through the pipes (that's Bernoulli for you). The second you stop flow the pressure downstream will equalize to the same pressure as upstream of your valve.
The only way to reduce pressure while water flows, and while it is not flowing is to insert a pressure regulator in the line. This is important since pipe joints may burst if subjected to the high pressure for extended time.
When the plumber said the valve reduces Volume I believe he meant volumetric capacity or better put, volumetric flow rate. A pressure regulator will cut flow rate to some degree, so look for a regulator with the highest Cv.