ᐅ Is a roof structure practical only with an underlayment membrane?

Hello everyone
We are building a bungalow, and the space under the (relatively large) roof will be used only for storage, accessible via a retractable attic ladder.
Different builders have proposed various roof constructions.
Our "favorite" roof structure consists only of the rafters, a vapor-permeable membrane, battens, and tiles.
They say this is best for allowing moisture to escape, but I’m not exactly sure where that moisture would come from.
In the house we currently live in, there are Gutex boards on the rafters, and the roof is built on top of those. That seems more solid to me.
But maybe I’m expecting too much.
What do you think?
Regards
RocketRider

Is this common standard practice...