Sustaining humanity would be the technical reason, but cmon no one actually cares about that. Maybe sustaining their own family name or some egotistical nonsense like that. The way I see it, there's very few good reasons to create more humans. One positive I could see be more social opportunities, but surely the spouse and friends should be enough. There is no reason one needs to experience having a child, and even then, I think adoption is better. Potentially helps those who's unfortunate existence is too late to prevent. Of course I would have a negative view on this given my upbringing, so perhaps that's keeping me from seeing the point.