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Well, one was if your brother dies without offspring — you had to take his wife even if you were already married and raise up offspring through her too.

Old Testament welfare. Women could not own property, so a woman's only security if her husband died was to have a father, a brother or a son. Otherwise, she would be destitute.So if your brother dies, your duty to his widow is not just to feed her today, but to give her a future. A son.

Everywhere and in every time before at least the 19th century, big families meant security -- it meant people to work the lands, daughters who could marry well, Sons who could land a bride with a dowry. And it made sense to have as many children as possible, since many -- if not most -- were likely to die of infectious diseases in childhood. Offspring were, to be blunt but honest, a form of wealth.

The old fantasy is of being the last man on Earth, with numerous women and a duty to replenish the species. In the hard-scrabble ancient days, a lot of civilizations and tribes were never far from that kind of threat of extinction. Polygamy is not a question of principle, but of pragmatism.

Men tended to die in work and war. Women in childbirth. A fertile woman not having kids was, again blunt but pragmatic, a finite resource going to waste. Sperm was a renewable resource. Women take time to gestate, and while waiting for that child to be born, a man can be out making more.

Look at any farm -- what's the ratio of bulls to cows, studs to mares, roosters to hens? Polygamy is like that. There's a reason that the art and science of efficient animal breeding is called husbandry.

Also, if you were married and caught with another woman (unmarried) you had to marry her too — and could never divorce her.

Old Testament child support. They didn't have paternity tests in those days' the way to hold a babydaddy responsible was to make him a husband, and thus the presumptive father.