This is actually a very good question, and one that troubled me for a while. A few years back one of my girlfriends got engaged, and we went out to celebrate. Sometimes during the night she started talking about what she "needed to change" about him.
I asked your question "Why are you marrying him if you want to change him?"
Her response was that he was about 75% of the way to being a good spouse, which was higher than anyone else had ever come. So, apparently she had a grading system or a scale against which she measured potential mates.
When I picked my jaw up off the floor I realized that most of the other women there agreed with her! OMG! Seriously? I immediately thought of where she rated on his scale, because she was pretty low on mine.
Maybe the other women were just trying to be nice, but I thought that maybe I was the weird one.
I used to have a shopping list when I was single. He was going to be tall, rugged, and handsome. I got the handsome part. Would I change him to be tall and rugged if I could? Not on your life! The rest of his 25% is just so full of surprises that I would have missed out on if I had stuck to my list.