Mostly for pet collectors to gather but some of the minions have interesting interaction animations with other minions if placed near each other. If you're a pet battle kind of person, they also have Verminion in Gold Saucer where you can join tournaments and use your minions to battle. read more
In patch 3.1, Lord of Verminion, a minion battle strategy game, was released. In the mini game, different minions have different stats and abilities. After using the minion whistles, players can summon their minions whenever they want by accessing the Minions Guide tab in the Character menu. You can only have 1 minion summoned at a time. read more