Why Object >> Mesh (edit) >> Mesh Data instead of grouped objects?
Most drawing programs support objects, which are primitive drawings that can be scaled, etc. Then they support grouping, which allows creating a tree of group objects, where each grouped object acts much like an object. This gives more flexibility than Blender, because the group object hierarchy may be arbitrarily complex.
Why was the Blender Object>Mesh>Data hierarchy chosen over the standard drawing program group object hierarchy?