It greatly simplifies life from a legal standpoint. It’s basically like creating a tiny corporation of two people that can act as a single legal entity. If you’re married it simplifies buying a house together, inheritance, medical decisions, etc. As others have pointed out, these are important especially when your partner’s family don’t approve of you or the relationship especially for LGBT people.

I am going to break the mold though and say the actual ceremony is important too. Declaring your intention to stay together for life in front of your friends and family changes things. It adds a level of security and finality to the relationship- you have to put your money where your mouth is on the relationship. Although people frequently do it, I don’t know how someone can go through the wedding process without reflecting on how big of a deal it is to stand up in front of so many of your friends and family and declare your intention to stay together forever, even without the religious ritual aspect of it. I wouldn’t want to have kids with someone without having this commitment, for example. Ultimately even though marriage is a social construct, I think it’s still a useful one even in a world where women are no longer considered property of men.