[idiotsecant]

That advice makes no sense but it is way easier to cook with dried mushrooms. Maybe that's where the folk wisdom came from. When you us mushrooms your goal is to remove as much mushroom juice as possible and replace it with fats oils etc. When you start off dried it's easier

[IAmBroom]

It does make sense; the poisons in morels (and many other "edible" mushrooms) are highly volatile. Heating them in a pan drives off the harmful compounds; heating them more gently in a large volume of liquid captures the compounds.

There's a variety of mushroom that has killed in the US, but is reportedly sold in Scandinavian markets. My theory is that Scandinavian recipes specify pan-frying or drying them first, and the unlike USian skipped this step.