Companionship. That's why I'd like to get married and what I miss about my relationship with my ex. I don't want kids, I don't much care about sex, but I'd like to love someone and know they love me, spend a lot of time with someone who's my absolute best friend, just plain have another person who's a part of my life. And somehow Platonic friends aren't quite the same, nor is my current situation of living with my mom.
There are times when I think I wouldn't even need to be passionately in love with someone--as long as we both really care about each other and get on well and are best friends, and as long as we're both 100% committed, that would make a happy marriage for me.
It's not so much that I'm all "OMG I AM ALONE WHAT SHALL I DO I MUST HAVE A MAN" as that I found out with my ex how nice it is just to have someone who's always there, and I miss that. I enjoy my solitude, but not ALL the time, and not at the expense of not having someone other than my relatives who cares about me. But then, I guess that's not really "for the sake of getting married"--i.e. being able to say that I am--it's more for the sake of the benefits that come of it. Which is really anyone's reason, I suppose.