The same use as the hundreds of species that lived and died before us that we at best are only marginally aware (made no impact, their lives had no "meaning"). The Saber Tooth Cat, the Tulle Elk, the Giant Cave Sloth, etc., populated the earth for the same reason we have...because we do.Certainly, it is a better outlook than every man for himself.
Still, if mankind is as small, short lived, and insignificant as must be the case if we just happened to evolve out of the primeval ooze all on our own, with no other intelligence helping with the process and no purpose behind it all, what does it matter whether we continue to populate this small planet or not? Of what use is human life?
We are all as flowers of the field; we live for a season and then we die. There has been nothing (save wistful thinking), that would indicate anything different.