Often it's tied into their fantasies about male dominance. Many of these players act out their real life frustrations in their sexuality, where they imagine having power over women in the traditional sense - power which they usually lack irl. And that includes ordering them into a specific role in life and society - to bear the children of the man who "dominated" them, since in their mind successful reproduction is only a sign of their power, as are the children that result from it and become part of their "property". The fetish is often tied to at least some part of that (pretty sick) dynamic. You can see the tendency when you compare the games on this site that do have pregnancy vs those that don't. Hope that answers your question.
I personally just like the prospect of having children, because I think life is beautiful.