I know I'm in high school, but I always liked answering these questions! (Sorry Jon)
I think married couples have children for many different reasons. I think some people have children because they love each other so much that they want to make part of that love something real, visible; something you can touch and interact with and help develop. I also think that having children brings some marriages closer together. When you have someone else (for instance, a small, adorable and utterly dependent baby) that you have to take care of, love, teach, admonish, and watch grow, you put some of your own needs and desires off to the side for that person, thus becoming a better and less selfish human being.
I'm also sure that having a child causes trust to grow between partners. It could be the simple "I trust you with our child" message you get when your partner needs to sleep, and hands off the kid to you. Or it could be the wonderful feeling of being needed when your spouse has his/her arms full and gives you a look or smile of gratitude as you take a screaming "bundle of joy" out of their hands.
I also think that having a child can ease tensions and stress from your everyday lives. Of course, there are moments when children can increase stress too, but overall I think there is nothing so gratifying as watching a kid's hilarious antics or rocking a baby to sleep on your shoulder as your spouse looks on. Children can inspire both laughter and tears, but all in all, I think they make married couples grow closer together and be a good deal happier because of them.