Glad you mentioned ‘next of kin.’ This is the important answer. If you’re married, you can do all that important legal stuff- make medical decisions if your partner is unconscious or indisposed, get the death certificate if that happens and give it to all the people who will need it.

Say your partner is in a car accident and you go to the hospital. There’s no marriage, no forms, no nothing to indicate you’re at all related to this person. You’re just some dude or lady, showing up at some dude or lady’s bedside. You can’t make the decisions for this person. Even if, say, they have a horrible narcissistic mother they’re estranged from- that mother, just by being the mother, can get all the authority to make decisions your unconscious partner would hate!

(Drawing from my own life. Fuck my mother.)

You can’t even call the hospital and get information on them. If they aren’t awake to indicate a release of information, the hospital can’t let you see them, can’t tell you anything.

This is just the first example that came to mind. The purpose of marriage is, it’s a legal way to indicate that you’re the most important person in the life of the person you marry. (And yes, depending on where you are and laws in your state or country or whatever, domestic partnership and other stuff can grant that, too.)