Possibly for water vapor that may form inside the wall and then needs to be able to diffuse out. This is apparently relevant in timber frame construction. It serves to protect the building. At least, that is what some carpenters and my architect have explained to me. It is by no means about allowing moisture to pass from the inside to the outside. You don’t want the moisture from inside to penetrate the framework, which is why a diffusion-inhibiting layer is formed between the interior and the framework. Usually with OSB, but there are other options as well.
The best wall construction doesn’t exist anyway. I assume that the building protection works well with a reputable provider. How one feels in the house – that is the decisive criterion. But how can you judge that when you haven’t built the house yet? Simply by spending time in other houses / show homes and focusing your attention on the sense of well-being. We have found that many new houses feel “stuffy,” as if you were in a Tupperware container. Such a wall construction is then simply not chosen.
No matter what you take, the difference is marginal. It's more a question of philosophy, with or without polystyrene.
However, with the variant using the wood fiber board, I would be careful that it does not "stand in water" at the base area. So, basically, you still need polystyrene at that spot.
