System families contain family types that you use to create basic building elements such as walls, floors, ceilings, and stairs in your building models.
Walls, floors, ceilings, and stairs are not families, they are reading and represented as the classes instances. Revit does not require loading a family for creation such elements. Family for example are windows, doors, columns, etc. For creation instances of such elements in Revit you need to import a Family in the file.