Adoption is actually the only reason for marrige I count as reasonable. I do not need a ring and documents if I know that I want to be together with someone (for the rest of my life), but should I want to have a child someday I most likely need to get married.
I can also see that it can have emotinal benifits for some. After all marriage counts as the highest status of relationships and it is still socially expected to get married, so doing it makes you feels less of an outcast. Not to mention that being in a relationship for a long time can be pretty hard and being married can give you more safety that your partner and you will continue staying together. At least that were some of the reasons why I wanted to marry when I was younger. ._.
When it comes to the topic of marriage, I actually like this video: