There are many exceptions, but cross culturally it forms the basis for reproductive unions and legitimizing offspring. Many anthropologists disagree and say it is an economic union because most of what married couples do cooperatively is not directly related to reproduction. This disregards the larger evolutionary context of mating systems. But they ultimately serve reproductive ends. You could not understand the union without understanding this.
Marriage is a contract between men, a formalized exchange of women as commodities.
Dowry often remains with the couple for their use.