I think in the US marriage is usually a tax benefit though not always. but that aside, i think it's mostly a lot of stuff about dying or divorce. inheritance rights, visitation rights in a hospital, power to make legal/medical decisions for the other person. there are rights if you separate. if you have kids there are probably issues there. it's harder to adopt if you're not married I believe.

there are responsibilities too, like if your spouse dies you inherit any debt they incurred after you got married (more or less, with exceptions, etc.)

ignoring the weight of tradition, the legal thing steps up your commitment. you can do it with contracts and wills and so on but assuming the kind of marriage you want is legal (e.g. not gay, at least here in california) i personally don't see the point of going that route if you actually want the legal commitment. you may as well do it the automatic way.

if there's a specific single thing you don't want the other person responsible for, you can make a contract for that, like giving power of attorney to someone else.

it's mostly the huge tradition / social norm thing. I mean i didn't exactly think about the legal benefits when i got married. but i'm glad to have them.