Why does it need to have a purpose? Parents have a duty of care towards their children and (usually) love them. That's motivation enough. Why make it more complicated than that?
It seems like a weird and rather amorphous goal, since there's never any way of knowing whether or not someone is the best person they could possibly be. I certainly doubt that many parents have this goal in mind.
>There are informed answers to that question.
To an extent. There's also lots of contradictory advice that's difficult to verify.