You create the subfolders and see them in the IDE; in the code you access them (read only) using File.DirAssets, all of them, without distinction by subfolders.
In DirInternal, which has nothing to do with this, you can create as many subfolders as you want.
I have no question.
I just think that no point to keep empty folder for assets.
I only agree if we have empty folder in DirInternal.
Usually I have empty folder such as "Uploads" or "Temp" in B4J project inside Objects (DirApp) folder. In this case, it makes more sense.