I am a much better person and a better writer having had my children than I would be otherwise. I would have missed a whole aspect of the human experience that's tremendously fundamental to things like characterization.
A lot of writers write as if the hero sort of popped out of the box at age 22 fully formed. And one thing that raising children does is give you some sense of how human beings really are put together. So when you go to put together a character you can have a more realistic sense of where people really come from, why they really behave the way they do and what a tremendous amount of life and complexity lies behind every human being.
But I think you can get that from being a father too. I think it's something you can do by growing up and being observant even if you don't have children.