A Folder is shared with a community and the community is "owner" of this folder. This is not a community folder but it is owned by the community and visible within the community.
If a user follows this community, the user is notified about modification in the community folder but not in folders that are shared with the community. This does not make sense from a community perspective and should be changed.
Use Case for this scenario:
You have one community but you need to grant editor access to a subset of files visible in this community, another group needs editor access to another subset of document. As for cloud customers the CCM component has been dismissed by IBM, this is a workaround which can be quite useful. The disadvantage has been explained above.