Well, it's very practical to marry your daughter off as soon as she's a biological adult(the start of puberty) if you live in a country without welfare.
If you live in a country without welfare you will have as many children as you can stand so that the chances of your children surviving is increased. Without welfare, it necessary for your children to take care of you when you're old. You also may want your blood line to survive. The government encourages this especially during times or war and famine.
(Around 680 B.C.)Large numbers of population died because of war or famine. In order to encourage childbirth, the government reduced the age of marriage to 15 for men and 13 for women.http://www.ncbi.nlm.nih.gov/pubmed/12285484
Without welfare, a woman requires a man to take care of her. Young inexperienced men can't compete with older more experienced men who may have also accumulated more assets. If you lived without welfare, you would not let your daughter marry an incapable male and risk having their entire family starve.
It's easy for Americans to not understand the practicalities of life outside of it's country, but is child-marriages really impractical or is America? http://www.usdebtclock.org/
Keep in mind, modern society's definition of pedophilia is dominant among all mammals including humans.http://egomoral.com/is-pedophilia-natural/