Here is one anthropoligst’s summary of the purposes of marriage:

To establish the legal (not necessarily biological) father of a woman’s children.
To establish the legal mother of a man’s children.
To define the rights of the husband, or his extended family over his wife’s sexual services. (the levirate–when brothers inherit a man’s wife when he dies)
To define the rights of the wife or her extended family over a husband’s sexual services. (the sororate–when a woman dies and her husband pairs up with her sister)
To define the rights of the husband and extended family over a wife’s labor power.
To define the rights of the wife and her extended family over the husband’s labor power.
To define the rights of the husband and his extended family over wife’s property.
To define the rights of the wife and her extended family over her husband’s property.
To establish a joint fund of property for the children.
To establish a socially significant relationship between the husband’s and the wife’s domestic groups.