Water vapor condenses into water and causes mold/rot. Closed cell foam doesn't let water out through the foam (open cell does, but open cell vs closed cell needs a long discussion of pros and cons - in general I'd call closed cell better if you can mitigate this issue). The problem is insulation is added in summer when there is high humidity (there is a minimum temperature) so all the wood is at the wetter stage, then winter comes and the whole wall cools off and any air trapped condenses the water since the water vapor cannot escape.