All the geometry created from brushes underneath a Model are stored in meshes stored in MeshRenderers/MeshColliders as child GameObjects of the Model.

Sub-Models would allow the user to create meshes for a subsection of the model by grouping a group of brushes under a submodel. The geometry of those brushes that would normally be added to the Model, would in this case end up in meshes stored with the submodel .

This allows the user to first just create the level without worrying about how to subdivide the geometry into sensible pieces for culling, streaming etc. and then later on add, or change the proper sub-models.