1) You can downgrade the firmware, even if you're 3.56+
2) You can play 3.60+ games straight from disk.
3) You can play backups the usual way trough MM if you're on 3.55DEX peek/poke patched
4) You can play 3.60+ backups if there was an update pkg released for that game.
(Max; 4 games to a HDD).
Those are the most relevant to the end user.
If you're a dev, than there are alot more goodies.