We don't really have a purpose, it's not like some supreme being created us. I guess if you really needed a purpose, it would be to continue reproducing and evolution, but you have to be adding quality genes to the gene pool...
I could bring so much religion into this, but i won't.
We are here just because organisms are able to survive in Earth's climate. If the organisms could survive on another planet, we would already be there, and the "we", as Nerdius put it, wouldn't really be "we" anymore.