WALL-E is the story of the last little robot on Earth. He is a robot and his programming was to help clean up. You see, it's set way in the future. Through consumerism, rampant, unchecked consumerism, the Earth was covered with trash. And to clean up, everyone had to leave Earth and set in place millions of these little robots that went around to clean up the trash and make Earth habitable again. (According to John Lasseter in a presentation to Disney corporate investors)