I think when the foolish monk asked "does a dog have Buddha nature?" and the wise monk said "Muu", he was right on the money.
In other words, "What is the purpose of human existence?" "Not applicable."
What's the 'purpose' of gravity, or purple? Is there a purpose to fire? Human beings are an emergent property of incredibly complex natural phenomena. There is no obvious purpose no matter how high or how low you go. You might say "the purpose of the universe was to produce humans", but that runs into questions of the anthropic principle and such chicken-and-egg problems. Smaller masses of matter orbit larger ones, energetic materials react, fizzle and burn, life exists as an emergent property of such natural systems.
"Life's purpose is to sustain its own existence" is a reasonable conclusion, but on the other hand people shove pool cues up their assholes and rip their guts up or whatever other mental malfunction like you can see across this very site, so I wouldn't be too convinced that there's a universal overriding purpose of the universe made with any actual intent.
If there really is a God, they would be so above us and probably unconcerned with us as we are with atomic quarks. Despite the principle of 'omnibenevolence', a deity would still have infinitely more free will than we do. Even the undeniable singularity of the human consciousness would seem more like an easily predictable and ultimately "meaningless" chemical reaction than a comparable mind worthy of anything but dismissal.
I find the deistic mode of thought more reasonable for what I see in the world. I think if there is anything larger than life or the universe, like a divine purpose, we've been set aside as a simulation (not to say a computer simulation, more like practice, like a prototype) and left to run to our conclusion. The data will be filed away to be used later as reference for the next one, but the actual contents of the test have no meaning. A single watch in a cabinet somewhere, ticking along until something breaks, to be analyzed later until the watchmaker gets it right, one day.
But does the ticking of the prototype watch itself, never to actually be used, have a purpose? Muu.
Either that, or it's the best reality TV show in the multiverse.
EDIT: typos