Ok it wasn't exactly like that. Or at least at first. He started off by saying it was needed for purpose of heaven.
Later as to my understanding. Wives were often shared to bare little ones. To bring up the population within the Mormon faith.
Later after Joseph Smith was killed and many of the men of the church were also killed.
Brigham Young became the leader of the faith. Or as they call it the Prophet.
When a wife, lost her husband by what ever means. He would take the widow to another man as his wife. And tell his wife to accept her as a sister