Your purpose is what you want it to be.
It is true that everything we do will eventually be rendered meaningless by the passage of time. The whole universe will cease to exist at some point, by which time mankind will likely be long gone. No one asks, what purpose did the Dinosaurs have? Why should man be any different?
Our actions will likely only effect a small group of people for a short period of time. Still, in the present we each must decide what and who we find important. What kind of person do we want to be? What effect will we have?