"Wofür hat man gute Familien?"
Also, "why is X true?" can be asking why something occurs, while "what for" is asking for purpose.
For example, Why does the sun rise in the East? is asking for an explanation based in astronomy, but What does the sunrise in the East for is almost metaphysical, like what is the purpose of the sun rising? A question that doesn't even make sense if you don't believe the universe was designed by an intelligent creator.
Why do we exist? can be answered with any number of explanations, including just explaining the science behind life.
What do we exist for? cannot be answered with the science. It necessitates inquiry into the meaning of life.