The main idea of the miracle system is, since faith is shared, to foster social interaction and cooperation between players on the same island. Suppose that you're a group of players on one island in one alliance, and you cooperate together to spread faith and upgrade the wonder. If a miracle that helps attackers/pillagers was coordinated during a pillage where every player from the alliance fired the miracle at an agreed upon time, all players can have the miracle coverage for an extended time that would cover the duration of the attack and well beyond.