For property purpose more generally: controlling inheritance was part of that, but a related part (and perhaps a more significant part in early societies where inheritances were small) was economic support the other direction, up the family tree rather than down. Where children (and grandchildren, and lateral relatives) care for their parents (and grandparents, etc.) in their old age instead of their existing nonfamily social support networks, establishing broader family bonds through formal unions is important to that.
E.g. many African tribes have traditions for polygamy that are either explicitly intended to deal with men being likely to die earlier (as fighters or hunters) to safeguard the viability of the house-hold and/or to deal with inheritance in societies where women were often not allowed to inherit (a typical case being a social obligation in some tribes for a man to marry his deceased brothers wives in order to ensure she still has somewhere to live).