I would say to push frontier of human knowledge forwards. Its not a high school anymore where you learn to learn. PhD is already a place where you should deliver value. (By "value" i don't mean commercial product)
> I would say to push frontier of human knowledge forwards.
Not sure if this is what you meant, but to be clear: pushing human knowledge forward is a practical prerequisite of a PhD (given it's pretty hard to convince people you're adequately trained to do such a thing without actually demonstrating it), but it's not the value of the PhD itself. The value is producing a researcher -- that is, someone who has the skills to continue accomplishing this in the future.
The difference is that if you happen to expand human knowledge by a stroke of dumb luck (or smart AI...) without actually having acquired skills to continue doing so in the future, then you're not really earning a PhD.
