Originally Posted by
noglider
A fabric can't let your moisture out and keep rain off if the air outside the jacket is much more humid than the air inside the jacket. That is the fallacy of so-called breathable materials.
But to be fair here, that's why they're good for winter riding. Cold winter air is very low humidity.
I think there might be something about heat involved in this as well, but you're right - it "allows" moisture to pass through the jacket to the outside, but if it's higher humidity outside it won't work.