Is roof construction only meaningful with an underlay membrane?

Hello everyone We are building a bungalow and the space under the (relatively large) roof is to be used only as storage space, accessible via a retractable attic ladder. Different builders have offered different roof structures. With our "favorite," the roof structure consists only of the rafters, a vapor-permeable membrane, the battens, and the tiles. That is said to be the best so that moisture can escape. But I don’t really know where it should come from. In the house we currently live in, there are Gutex panels on the rafters, and construction was done on top of that. Looks more stable to me. But maybe I’m expecting too much. What do you think? Regards RocketRider

Is common standard like this...