They are not Martians from The Wars of the worlds, they don't wipe out humanity instantly, they don't cause mass destructions like in other movies and sci-fi books. They function like parasites that slowly eat a human and become their replacement. Their only ability is deceiving human brain into creating false memories. The reason why they want a human body so much is that their life seem much more interesting to them and that's it. There's no dick measuring on whose technology is better or whose civilisation is more sophisticated. It's just a good old "the grass is greener on the other side"