Marble uses Qt's Model/View framework as a way to signal updates to a model to interested parts. In that respect, a Model class can wrap any possible data.
Here is the list of Model class, and a description of how they are used.
It represents a data file, parsed into a GeoDataDocument tree structure.
It is used by the DataViewPlugin debug plugin (and is broken atm)
It represents a list of gpx data file represented as GpxFile
It is used by the GpxLayer to load files and access them (and it doesn't look like it needs the Model-specific methods)
It is used by the GeoRenderPlugin to access the list of placemarks.
It is used by no other code.
It is used by