From a purely scientific standpoint: there is no purpose. Fundamentally, life exists because it can. It reproduces because only life forms that reproduce survive. Wrecking the biosphere is "hard coded" in the sense that we, are a species, are selfish and short-sighted. We just happen to be more capable than other species, we have technology, and the means to radically transform this planet over relatively short time spans.
Over a longer time horizon, the strategy we're employing might backfire on us. As every other species, we'll have to adapt or perish. Overpopulation and the destruction of our own habitat are not viable. I'm hopeful because solar energy and battery storage technology is improving rapidly, while also getting cheaper. We are definitely causing some damage, but ecological awareness is also improving. We have a decent shot at reaching some kind of equilibrium where we take better care of our planet, so that it can sustain us.