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.
If people are doing bad job at keeping themselves and their loved ones well, why not help them with little scientific knowledge?
Why doom them to do only as well as what they learned from their parents allows them to?