Well, one good thing about marriage is that if one of you is employed the other can get health care from the other person's company. Or one can be the employee with benefits and the other can be the contractor with higher rate-per-hour because benefits are provided by the employee.
Another good thing is the status each of you gains in the family of the other. So that through marriage you both gain access to a larger family network.
I also think monogamous relationships in general are a good compromise relationship so that both partners can feel an equal investment in supporting the offspring as well as safety from one partner contracting STDs. Although I have to admit being intrigued by polygamous and polyandrous marriage arrangements.
I think the standard marriage contract is not very good and a better contract could be negotiated by both parties before entering the marriage, but I was too lazy to do this myself.