It?s simply a word for convenience, since in our society, Theism is the norm and everyone pretty much assumes by default that you must be a Theist.
Maybe there shouldn?t necessarily be a word for Atheists, and in a perfect world, there wouldn?t need to be. But, to paraphrase MovieBob, it?s not a perfect world. It?s a shitty world, and this kind of stuff still happened. Some people believe that heterosexuality isn?t the default or ?normal? sexuality, yet we still have labels like ?homosexuality?, ?bisexuality? and ?asexuality? to describe people that don?t fit the supposed majority.
Labels are labels. We humans need to put names to things, otherwise we go insane wondering how to categorize certain minorities.