To have children? To reproduce and keep mankind going of course!
I never have children or even a girlfriend so my worldview is limited. I am not yet an adult yet....but in about a few years or so I will enter adulthood.
To me, I think having children is to have a family. Somebody to love and care for. Because you create them I guess. And perhaps embodies your wishes and dream that you will not attain in your lifetime.
I think this is the reasons people have children. I don't know really if this is the reason because I never have children myself.