Seems to be a status thing to see how low one can idle their engine. And another status symbol being a big engine many idle their engines very low. Especially around docks and floats where people can hear what a big monster engine is in the boat.
All this is really hard on damper plates and more importantly springs. Best for damper plates is to prop your boat to rated rpm and adjust idle speed up to where the engine runs reasonably smooth. One to 200rpm above the usual does absolutely no harm. Well if you’re over propped a low idle speed will be almost necessary. But it’s not good for damper plates.
North Western Washington State USA