This is kind of a lazyweb question because the "why marriage?" question has been discussed many times before on HN.
https://www.google.com/search?q=marriage+site%3Anews.ycombin...Beyond the social convention that "it's the thing to do" at a certain point in a relationship, some reasons are:
- Marriage provides financial assurance for a spouse who will defer their career to raise children.
- Some friends and family (particularly those of the spouse) will absolutely treat you differently when you're married to someone versus in a long-term unmarried relationship.
- Depending on the jurisdiction, spouses have certain rights that unmarried partners do not, e.g. with regards to the legal system and hospital visitation. I know a couple that got married because one of them was an activist/journalist and frequently attended protests in which he was at risk of arrest.
- In some circumstances, it makes it easier to purchase a home together and make other large joint financial investments.
- Under some tax regimes, you pay lower taxes when married.
- U.S. citizens living abroad in a low-tax country with a foreign spouse can engage in advantageous tax planning.