This is the title of the second essay in Kay Hymowitz' Marriage and Caste in America.
The answer is "raise children." The distinctive feature of American marriage, compared to the Old World models against which we revolted, is that couples were much more supported in choosing who they would raise children with. A republic needs civic-minded, self-reliant citizens, and the best nursery for such adults is a strong democratic family. What has changed since the cultural revolution is that children are being driven out of the picture. Marriage has been redefined as being about the happiness of the couple, not the protection of their children.
The good news in Hymowitz' essay is that this redefinition of marriage may not be a permanent change, but is another idiosyncrasy of Baby Boomers. As they age out of childrearing, the generation on the front lines gets to try to restore the main task of marriage to the center of the job description. She cites a Yankelovich survey that found a bare majority of Boomers supported a return to more traditional, child-oriented standards of family life -- whereas nearly three-quarters of Gen Xers do.
It will be a hard slog to restore democratic childrearing as the first job of families. But it will be worth it, because that is what American marriage does.