Book 15 says earthly families are the production plants of heaven’s citizens. The point is not bare fertility but that heavenly people are formed through embodied family life on earth rather than appearing in abstraction.
That gives public weight to household life. Marriage, parenting, and generational culture are not side issues to spirituality because citizenry is generated there.
The claim is useful because it makes the family more than a private comfort zone. It becomes part of the providential infrastructure of God’s Kingdom.