According to Vonnegut, man exists to one day make a small piece of metal and deliver it to a moon of Jupiter.
According to Zarathustra (the Nietzchean hero, not me) we exist to become enlightened.
According to me, we exist to endlessly debate the reason for our existence.
If existence has a reason, mankind sure as hell isn't high enough in galactic foodchain to be privy to the secret.