This is more of an origin question IMO, because knowing why we are here is linked to knowing how we got here. If you look at nature on Earth, every living thing has its "niche" or place where they seemingly are meant to be in the cycle of life. Humans do not have this niche. No other known species builds up, they live in nature and hunt. It makes you wonder if humans really belong on this planet.
Knowing why we are here might be as simple as something putting us here. A far advanced race in ancient times could have found this planet and made us to be put here. Hell, an almighty God could have thought us into existence, but we will never know for sure unless it comes down and tells us.
Scientists are able to make clones in labs, such as sheep, mice, even humans. If one is to clone a living thing, and this living thing has conscience thoughts, what is its purpose? Could it be satisfied with the fact it was made as an experiment in a lab, or can it have a God? Would the scientist be its "God" since it is the creator?
With any conclusion, yes we know there are billions of people who live and die, and among space earth is but a spec of dust, but isn't that a reason to be happy? You know that there is nothing you can do to fuck any of that up, so do what you want in life and make your own purpose.