In some societies people want children for what might be called familial reasons: to extend the family line or the family name, to propitiate the ancestors; to enable the proper functioning of religious rituals involving the family. Such reasons may seem thin in the modern, secularized society but they have been and are powerful indeed in other places.
In addition, one class of family reasons shares a border with the following category, namely, having children in order to maintain or improve a marriage; to hold the husband or occupy the wife; to repair or rejuvenate the marriage; to increase the number of children on the assumption that family happiness lies that way. The point is underlined by its converse: in some societies the failure to bear children (or males) is a threat to the marriage and a ready cause for divorce.
Beyond all that is the profound significance of children to the very institution of the family itself. To many people, husband and wife alone do not seem a proper family they need children to enrich the circle, to validate its family character, to gather the redemptive influence of offspring. Children need the family, but the family seems also to need children, as the social institution uniquely available, at least in principle, for security, comfort, assurance, and direction in a changing, often hostile, world. To most people, such a home base, in the literal sense, needs more than one person for sustenance and in generational extension.