Not an expert, but there is a lot less ground to legally challenge the partnership if a couple is married. If a couple isn't married and one of them suddenly dies without a will or some sort of similar situation, the deceased's family can fight for everything from how their funeral is executed, custody of children, or even control of finances. Obviously it would involve some sort of legal intervention, but marriage would supercede any familial relationships in most cases. I'm not sure how common it is, but I've definitely heard stories about this in gay relationships where estranged family ends up getting legal guardianship of children because the parents weren't officially married and one died.

I'm married and, though I don't think I can numerate all the ways, we are definitely more financially stable than we were when we were just dating. Even just the convenience of being able to do certain legal and medical stuff on my partner's behalf makes a big difference. And that's isn't to say it's for everyone, but it is worth investigating if you have specific concerns.