I think looking across human history, a mostly independent, nuclear family is clearly not the only way structure a functional society and raise children. In some respects, it seems unfair for modern parents to have the shoulder the most of the burden of raising their children by themselves, when in other societies there might have been a supportive village, tribe, or clan house to share the responsibilities of parenting. Plus, the current system makes it disastrous for the children if the married couple breaks up (something that might have been less disruptive in other arrangements with more caretakers available). It seems like it would be more fair to children if there more communal support available.
Conversely, though, modern marriage is clearly more egalitarian and fair than polygamy, emphasizes the individual's right to select a mate, and generally promotes social stability. So, I think in the US it adds value and is an institution worth supporting.