Why would a grown man want to marry a 9 year old little girl?

I have just read the book, "I am Nujood, Age 10 and Divorced."  I saw this book in the book stores a couple of years ago.  Now I have decided to read it.  Long story short this book is about a little girl whose father arranged for her to get married when she was 9 years old.  There are many factors which would drive a father to marry off their daughters at such a young age: poverty, local custom, and lack of education.  Nujood's family is from Yemen and marrying a young girl has been going on there for generations.  There is a tribal proverb that says: "To guarantee a happy marriage, marry a 9 year old girl." By the way, Nujood's husband was 30 years old.  What can a grown man have in common with a child of nine? I felt very sad after reading this book and nearly cried.  I just don't understand the thought pattern here and I kept asking myself why. I know that they want to ensure that children will be born to them in the marriage but oftentimes at the peril of the mother.  A child will not be as developed internally as she should and faces serious injury or worse.

Picture comes from National Geographic

Nujood shattered a taboo. This child's story traveled around the world.  Her courage to seek a divorce from an abusive man three times older than she, gave other girls the courage to stand up for themselves too. In February of 2009, the Yemeni Parliament finally passed a new law raising the legal age of consent for marriage to 17 for both boys and girls.  I wonder if this law is enforced?  I doubt it very seriously.