PortalContentFolder (for CMF) is a mix of both PortalContent and SkinnedFolder, so that you can have a folder-like object that gets catalogged and is subject to the workflow.
If you need an object that has to be catalogged (because it has to be found in a Search, or is to be displayed using searchResults) or workflowed, and at the same time have subobjects, then you should use this.
PortalContentFolder is a base class that can be used as a Factory-based Type in the Types Tool. It can also be used as a base ZClass.