Only reason you need: It exists in most places and is so ubiquitous that you will probably never get rid of the concept of marriage no matter how hard you try, so you might as well live with it and work around it. Maybe it's useless to you, but a lot of people like to do it. It's like asking why some cultures still eat really unhealthy(but hella delicious) stuff when they have access to organic cow-fed vegetables. Because it's their culture they always had, and it's just really delicious.