The Gameboy Color version of Dragon Warrior III has an exclusive feature – link cable support.

If two copies are connected with a link cable, Monster Medals can be traded. Up to three medals can be swapped at a time and uneven amounts are accepted. Meaning, Player 1 can send one medal while Player 2 can send 2, as an example.

However, these medals do not serve a purpose unless you manage to grind for them all. Only then will the post-game dungeon and boss become available. So trading can help make this process a little easier if you link with another dedicated DWIII player.