For all societies before about 1870 at the very earliest, the reason for large families was the very high death rate: a typical woman could expect to get pregnant 10 times, lose five of them to miscarriage, lose two of the five live births before their first birthday, and another at some later point before adulthood. These families typically would be so close to the food margin that the rule of thumb was mothers expected to lose a tooth for every live birth- the baby and the mom would struggle for the same calcium. This is a serious deal, and the main reason for the large families- to survive all of that death.

And even the wealthiest families would still lose kids. In 1924 the 16 year old son of the President of the United States, Calvin Coolidge Jr, went out to play tennis against his brother. But he forgot to put on his socks. He got a blister, it popped, got infected, and he died. That was the world that humans lived in just a century ago, even the most powerful could expect to lose children before adulthood. And so they needed large families to ensure that some of them made it.