subpanel ground requirement - attached vs unattached structure - what difference does it make?
There's no longer any difference, aside from the fact that a detached structure needs to be bonded to a separate grounding electrode. In both situations you're now required to run 4 wire cable, and keep the grounded and grounding conductors separate in the second panel.

A detached structure has to be bonded to a grounding electrode, to reduce voltage transients between the structures. This is because the ground "earth" can be at slightly different electrical potentials, even over small distances. Bonding the electrode to the grounding system of the main structure, insures the entire grounding system is at the same potential.