I'm not an expert on the subject, but this is what I consider to be an educated guess on the matter:
1. For distribution of chores. They had to get up at dawn, or in many instances before dawn, and work until the sun went down. Lots of work and long hours. I'm guessing if you had more kids to do the chores, then they'd all get done by the end of the day, and with a little less exhaustion.
2. High infant/child mortality rate. Perhaps those families with zero kids had lost them by the time of the 1790 census. It's also possible that there were adult children who had established their own farms, so those who were listed without children were older couples with grandchildren. There is also the possibility that, for a variety of reasons, that in an era of bloodletting and general lack of medical knowledge, there were a lot of miscarriages and stillbirths. It's likely some couples were never able to have children.
3. The high infant/child mortality rate also explains the need to have large numbers of children. If you have ten, you could expect to lose maybe five in their first five or ten years of life. That meant you still had five kids left to help run the farm. I know it sounds a little heartless, but having lots of kids to "make up" the loss was probably considered practical and realistic back then.
4. Religious and societal expectations. Having lots of kids conforms to the Biblical commandment to be fruitful and multiply. Women were pushed into rigid gender roles where their primary purpose was to have children, and also to obey and submit to their husbands, being their property. There was no birth control*, other than complete abstinence. And during those long, cold hard winters when there was little to do but sit inside by the fire, read the Bible and tell stories, once the kids were sent to bed, the parents stayed warm with some nookie. Nine months later, another kid.
*At least no officially sanctioned birth control.