There are far more technical experts on this board than me, but I think that the clay pot thing is about the rate of evaporation that occurs in those vessels, which in turn can affect the flavor of the item cooked (positively).
Now. I have also had a clay pot add a wet-clay favor to the food cooked in it. Which was not positive at all, in fact it was downright nasty.
Similar with cast iron. When I get an actual flavor added to the foods cooked in my cast iron, it is an awfulsome metal flavor.