Hi, I wondered about this myself. I couldn't see much of a difference in the demo code, in the way it worked. My first thought was that a model was a way to make clones... like make a model then clone it to make new items, but I don't think that is it.
If you look at the docs that come with the Goo c library, the Model class is designed to make subclassing of items and designing your own items simpler. I don't grasp it all yet, but it seems that the various Model classes are the base classes of the various item classes (which are more complex than the base model). There are subtle differences in the inheritance heirarchy between the model and the item, and that probably make it easier to use it as a base class.