My sons just make me a more well rounded person. They teach me things through their eyes, and help me to keep my priorities straight. It is easier to stay on the hard road when you have people behind you that count on you. When you are really sick and can barely hang on they give you the courage to fight and win.
I do not view my sons as serving a purpose because I do not own them. They are just two men I brought into the world who I helped to shape in some way for better or worse. However, they are the only thing I can leave on this planet since they carry on my DNA, so if they do serve a purpose it is survival of our family name.