Marriage is the primary institution around which culture is organized. It organizes inheritance, childrearing and sexual behavior (for "adultery" is still illegal in many places). It even organizes fantasy since it is regarded, generally, as the proper outcome of romance. Marriage is also part of religious coherence and love between marrieds is considered sacred, whereas love that is unbounded by marriage is considered sinful. Thus marriage regulates pleasure.
So asking why it is "important" is like asking why the church is important. If you are raised in the institution, it becomes part of your values. In this case, the adoption of the value, even when you are dubious about it, has a financial incentive in terms of tax benefits. (And divorce law effectively punishes you for fucking up the sacred institution.)
I hope it's clear I don't support marriage but it's pretty obvious why it's important to so many people.